Portia Kapraun has always seen unwelcome ads on Twitter, usually from major brands pitching her luxury jewelry or vehicles that she, as a librarian in Indiana, could not afford.

But the mix now is far more annoying: More ads for random gold investments, she said, and also a badly designed ad for what looked like a tabletop foosball set constructed with rubber bands and particle board, which promised its product would be the most fun family game she had ever played.

Ms. Kapraun was not interested. But she soon saw the ad again. And again. And again.

“I don’t know who they think I am, but that did not look especially fun,” she said of the foosball set. “These feel like bargain basement advertisers. It mostly seemed like things you would see if you were watching late-night television.”

In a shaky advertising market in an uncertain economy, ads that few people want to see suddenly seem to be everywhere.

Recent ads on Twitter, as described by users, have made the platform feel like a tabloid magazine or the haunting ground of Ron Popeil, the inventor of wares people didn’t know they needed including the Veg-O-Matic, the Ronco Electric Food Dehydrator and the Inside-the-Shell Egg Scrambler. There were ads for T-shirts printed with a horse’s head superimposed on a heartbeat line, served to someone who does not ride horses nor particularly like them. Also: fraudulent ads for discount drones, spots hawking crude gaming apps and promoted posts from people ranting about “puppet masters” and “the slave mind.”

On Instagram, ads from Amazon promote unrecognizable contraptions from obscure merchants, echoing the shopping site Wish, which was notorious for bizarre ads. On YouTube, ads impersonate popular video creators to scam viewers, a phenomenon that has irked Elon Musk and that YouTube says it is addressing.

Advancements in digital advertising technology were meant to improve users’ experience. People interested in shoes are intended to get ads for sneakers and loafers, and not repeated pitches for courses teaching seduction techniques. And the technology is supposed to filter out misleading or dangerous pitches.

But lately, on several platforms, the opposite seems to be happening for a variety of reasons, including a slowdown in the overall digital ad market. As numerous deep-pocketed marketers have pulled back, and the softer market has led several digital platforms to lower their ad pricing, opportunities have opened up for less exacting advertisers.

“Anytime you lower the barrier to entry, you’re going to get lower-quality entrants,” said Jessica Fong, an assistant marketing professor at the Ross School of Business at the University of Michigan.

In the past, buying a newspaper or television ad usually involved calling up a representative who would manually review and place the ad. Now, more than 90 percent of spending on digital display ads happens through automated software.

Social media offers many ad formats — static text, videos, playable games, messaging, brand takeovers, custom filters — and most are getting easier to buy. Many advertisers can now go online and set the budget for how much they are willing to bid through a platform’s automated auction for a spot that reaches their target audience.

Evidence of a junk-ad epidemic is anecdotal; tech platforms rarely reveal data on who advertises with them and how often. Also, quality is in the eye of the beholder — plenty of people willingly click on male enhancement ads, and ads are sometimes most successful when they are eye-catchingly terrible.

Social media platforms have said they have established rigorous advertising policies to safeguard standards and continue to attract first-rate ads from blue-chip companies.

But advertising experts agree that crummy ads — some just irritating, others malicious — appear to be proliferating. They point to a variety of potential causes: internal turmoil at tech companies, weak content moderation and higher-tier advertisers exploring alternatives. In addition, privacy changes by Apple and other tech companies have affected the availability of users’ data and advertisers’ ability to track it to better tailor their ads.

Then, there’s the economy: A survey of 43 multinational companies representing more than $44 billion in advertising spending, conducted last fall by the World Federation of Advertisers, found that nearly 30 percent planned to shrink their marketing budgets this year. Clorox, which budgets hundreds of millions of dollars a year to advertising and promoting products like Burt’s Bees lotions, Brita filters and Pine-Sol cleaners, said this month that it was beginning to streamline its marketing, which included cutting back on spending.

Digital ad spending, while still growing overall, “has decelerated precipitously,” according to an analysis last month by the research firm Insider Intelligence.

Twitter seems to be faring the worst. The company has struggled to retain top-flight advertisers since Mr. Musk took over as owner in October, amid fears of a proliferation of hate speech and misinformation on the platform. Its 10 largest advertisers last year spent 55 percent less during Mr. Musk’s tenure than they did a year earlier, with six of them spending nothing so far in 2023, according to estimates from the research firm Sensor Tower. Twitter has offered buy-one-get-one-free deals, discounts and bonus incentives to lure back advertisers, media buyers said.

But advertising troubles have hit the biggest publicly traded social networks, too. Snapchat’s parent company last month posted its slowest-ever rate of quarterly growth and projected a sales drop for the current quarter. Google’s parent company, Alphabet, said ad sales at YouTube slipped nearly 8 percent in the latest quarter.

Last year, Meta, which owns Facebook and Instagram, reported its first decline ever in quarterly revenue (it fell again last quarter). Ad prices on Facebook and Instagram fell 24 percent in the last quarter of 2022 from a year earlier, according to the investment bank Piper Sandler.

Shareholder pressure, stoked by years of big profits, continues to push those companies to generate revenue wherever possible  — including, experts said, through selling low-quality ads.

Corey Richardson, vice president at the multicultural ad agency Fluent360 in Chicago, said he was seeing more ads for items in which he had no interest — Hawaiian shirts featuring “Star Wars” characters, a fountain shaped like hands folded in the prayer position, all mixed in with vaccine misinformation and the occasional video depicting violence.

“They’re just taking whatever money comes — beggars can’t be choosers,” Mr. Richardson said.

Twitter did not respond to a request for comment. Meta declined to comment. YouTube said it invested “significantly” in ad quality and consumer experience.

Other factors are also contributing to lower ad quality, Mr. Richardson said. Social media advertising, once a niche art practiced by specialist agencies, is now easily available to anyone. Many of them are eschewing targeted ads — placements intended to reach specific audiences, usually at a higher cost — in favor of a cheaper spray-and-pray approach online, hoping to catch the attention of gullible or bored shoppers.

Major social media platforms are now “like the mall in your hometown that used to be the good mall,” Mr. Richardson said: “But now there’s no longer a Macy’s there — it’s just a calendar store and a place selling beepers.”

The ads are also increasingly inescapable. Twitter recently revoked free access for third-party services like Tweetbot and Twitterific, which helped many users avoid unwanted ads. A project that blocked YouTube ads was forced to shut down in March; months later, YouTube said it concluded a small experiment that tried to reduce ad breaks by moving more ads into fewer interruptions, sometimes showing viewers as many as 10 unskippable ads in a row. In the fall, a survey by Insider Intelligence found that 52 percent of Facebook users reported seeing more ads on the platform, compared with nearly half of YouTube and Instagram users.

Meta allows users to personalize their ad experience, changing their preferences when they see an ad they are uninterested in. The company has also said it does not approve ads about subjects like climate change that its fact-checking partners have categorized as false, and it takes down ads that violate its policies.

In the past year, researchers and journalists on Facebook have discovered ads for dangerous or illegal dietary supplements; dozens of since-removed ads showing photoshopped images of the Australian prime minister that linked to investment scams; and allegedly misleading advertising timed to the COP27 climate summit. After being contacted by The New York Times, Meta took down Instagram ads that ply cancer patients with polished pseudoscience promoting vague but expensive alternative treatments.

YouTube said it prohibited ads that impersonate businesses, swindle users or employ clickbait tactics. The company said it blocked or removed more than 3.4 billion ads and suspended more than 5.6 million advertiser accounts in 2021.

“Though there are always bad actors looking to bypass enforcement systems across all tech platforms, we remain vigilant in trying to identify and remove them as quickly as we can,” YouTube said in a statement.

Beyond social platforms, bad ads appear elsewhere on the internet. But streaming services and news websites tend to have stricter advertising guidelines and more limited and expensive ad space, which makes ads easier to regulate. Some publishers, like Bloomberg Media, are starting to avoid third-party brokers and automated auctions of ad space, which deal with enormous volumes and are more likely to miss low-quality ads.

Social media is a far easier target for the small but motivated group of anti-vaccine advertisers studied by David A. Broniatowski, who helps run the Institute for Data, Democracy and Politics at the George Washington University. Their ads are designed to evade “very brittle” moderation algorithms by spacing out the letters of banned keywords or replacing them with emojis, he said.

“They will use whatever means necessary to get their message out there,” he said. “Ads are simply one tool in their toolbox.”