Elon Musk's tweet saying his planned Twitter Inc. acquisition was on hold put the spotlight on questions about fake accounts, which have vexed social-media platforms for years.

Mr. Musk, the chief executive officer of Tesla Inc., said in a tweet early Friday that his $44 billion deal to buy Twitter was "temporarily on hold pending details supporting calculation that spam/fake accounts do indeed represent less than 5% of users."

Spam and fake accounts are an industrywide challenge. They can hurt the experience for legitimate users, who could see posts that they don't realize are generated by computer programs or otherwise under false pretenses. Operators of "bots" have used automated fake accounts to incite violence, spread false information, deceptively attempt to influence political activity or achieve other illicit goals. And fake accounts can make it harder for advertisers on social-media platforms to judge what they are getting for their money.

Twitter and the companies behind rival platforms such as Facebook and YouTube have spent years trying to stamp them out. The companies have expanded content-moderation teams and used artificial intelligence, but many describe spam and fake accounts as a whack-a-mole problem since bad actors are constantly updating their methods.

Mr. Musk -- in challenging Twitter's calculations on spam and fake accounts -- provided no evidence that the company's disclosures on the matter have been unreliable. In the tweet saying his deal for Twitter was on hold, Mr. Musk included an article referring to a recent regulatory filing from Twitter in which it reiterated that such accounts represented less than 5% of its monetizable daily users in the first three months of the year. Mr. Musk later added that he remains committed to the acquisition.

Twitter's filing repeated previous statements, including that the 5% figure is based on a sampling and that "the actual number of false or spam accounts could be higher than we have estimated." The company, which had about 229 million daily active users in total as of March 31, hasn't said how active the bogus accounts are.

Late Friday, Mr. Musk tweeted again, saying he would try to check the figure and urged others to do the same: "To find out, my team will do a random sample of 100 followers of @twitter. I invite others to repeat the same process and see what they discover."

Mr. Musk then, in response to questions, spelled out ideas on how to proceed, tweeting: "If we collectively try to figure out the bot/duplicate user percentage, we can probably crowdsource a good answer."

Mr. Musk, asked on Twitter whether he had thought about the issue before making his $44 billion purchase commitment, said: "I relied upon the accuracy of Twitter's public filings."

Twitter's disclosures about the share of spam and fake accounts have been consistent for years, and the company has outlined measures it takes to address the issue. In its latest transparency report, a roundup of user-data and other statistics that many big social-media companies issue on a regular basis, Twitter said user reports of spam rose nearly 10% to 5.1 million in the first half of 2021 from the second half of 2020. It said users submitted 29.8 million spam reports between January 2018 and June 2021.

Twitter hasn't disclosed details about the impact of spam and fake accounts on its platform. "They could be tweeting just once a day or a hundred times," said Joseph Seering, a researcher at Stanford University who studies content moderation.

Mr. Musk's remarks added to speculation that he might try to negotiate a lower sale price or walk away from the deal. In the midst of a broader market pullback, Twitter's shares are now trading below where they were when he made his unsolicited offer for the company last month. It couldn't be determined what role, if any, that had in his early-Friday tweet or why he is now raising the issue of phony accounts in relation to the transaction.

Mr. Musk has previously flagged the presence of spam and fake accounts as a concern. As part of his bid to buy the platform, he tweeted that such accounts harm user trust.

Write to Sarah E. Needleman at sarah.needleman@wsj.com