Shares in Twitter have plunged after Elon Musk put his $US44-billion ($64 billion) takeover bid on hold, just weeks after agreeing to take the company private.

Shares of the social media company fell 17.7 per cent to $US37.10 in premarket trading in the US on Friday.

It was their lowest level since Mr Musk disclosed his stake in the company in early April and subsequently made a “best and final” offer to take it private for $US54.20 per share.

The implied probability of the deal closing at the agreed price fell below 50 per cent for the first time on Tuesday, when Twitter shares dropped below $US46.75.

Tweet from @elonmusk

It came after Twitter said days ago that false or spam accounts represented fewer than 5 per cent of its monetisable daily active users during the first quarter, when it had 229 million users who were served advertising.

The company said the estimates were based on a review of sample accounts and it believed the numbers to be “reasonable”. But it acknowledged they were not independently verified, and said the actual number of fake or spam accounts could be higher.

“Twitter deal temporarily on hold pending details supporting calculation that spam/fake accounts do indeed represent less than 5 per cent of users,” Mr Musk tweeted on Friday.

A couple of hours later he followed up with: “Still committed to acquisition.”

Mr Musk, the world’s richest man and a self-proclaimed free speech absolutist, had said that one of his priorities would be to remove “spam bots” from the platform.

Twitter did not immediately respond to a request for comment. Neither Mr Musk’s representatives nor his company Tesla Inc were immediately available for a comment.

But some analysts were critical of the decision, and the way Mr Musk announced it.

Tech analyst Dan Ives of Wedbush Securities told clients in a note that the billionaire had turned “this Twitter circus show into a Friday the 13th horror show”.

Cancelling the deal would cost Mr Musk $1 billion.

“[Wall] Street will view this deal as 1) likely falling apart, 2) Musk negotiating for a lower deal price, or 3) Musk simply walking away from the deal with a $1 billion breakup fee,” Mr Ives wrote.

“Many will view this as Musk using this Twitter filing/spam accounts as a way to get out of this deal in a vastly changing market.”

Most US stocks, especially in the tech industry, have fallen markedly in the time since Mr Musk announced his deal.

Josh White, an assistant professor of finance at Vanderbilt University and a former financial economist for the US Securities and Exchange Commission said decisions such as Mr Musk’s were usually revealed in official filings.

“This happens as you get access to the books and access to proprietary information. What doesn’t normally happen is a tweet,” Assistant Professor White said.

The unusual move could draw the attention of Twitter’s lawyers, he told CNN.

As part of the deal, Mr Musk agreed to consult Twitter before making any public statements about it – and to avoid tweets that “disparage the company”.

If the deal falls apart, “I would expect Twitter’s current shareholders to potentially bring a lawsuit”, Asisstant Professor White said, adding that Mr Musk’s actions had damaged them by tanking the stock price

Twitter said earlier that it faced several risks until the deal with Mr Musk was closed, including whether advertisers would continue to spend on Twitter amid “potential uncertainty regarding future plans and strategy.”

Mr Musk has been critical of Twitter’s moderation policy. He has said he wants Twitter’s algorithm to prioritise tweets to be public and was against too much power on the service to corporations that advertise.

Earlier this week, he said he would reverse Twitter’s ban on former US president Donald Trump when he bought the social media platform, signalling his intention to cut moderation of the site.

-with AAP