The crisis at Twitter reached new heights after hundreds of employees were reported to have rejected Elon Musk’s ultimatum to keep working for the business, threatening its ability to keep operating.

As the company temporarily closed its offices to staff on Friday, Twitter users began saying their goodbyes and linking to accounts on other platforms.

#RIPTwitter, #TwitterDown, Mastodon and Myspace were all trending on the platform after the deadline passed on Musk’s ultimatum for the remaining workforce to sign up for “long hours at high intensity” by Thursday, or leave. It has been estimated that hundreds of the remaining staff, already cut from 7,500 to around 3,750 in the wake of Musk’s takeover of Twitter last month, opted to go.

The departures include many engineers responsible for fixing bugs and preventing service outages, raising questions about the stability of the platform amid the loss of employees and prompting hurried debates among managers over who should be asked to return, current and former employees said.

In an early sign that the number of those declining to sign was greater than anticipated, Musk eased off a return-to-office mandate he had issued a week ago, telling employees on Thursday they would be allowed to work remotely if their managers asserted they were making “an excellent contribution”.

Twitter later announced via email that it would close “our office buildings” and disable employee badge access until Monday, the New York Times reported.

Online, users were speculating that the site could go down in a matter of hours or days. In July Twitter’s former head of security, Peiter Zatko, filed a whistleblower complaint warning of poor infrastructure standards at Twitter even before Musk’s takeover, alleging that more than 50% of Twitters 500,000 data centre servers are running software that is out of date or have other known security problems.

On Thursday evening, the version of the Twitter app used by employees began slowing down, according to one source familiar with the matter, who estimated that the public version of Twitter was at risk of breaking during the night. Website DownDetector reported a significant uptick in user reports of issues on the site.

“If it does break, there is no one left to fix things in many areas,” the person said, who declined to be named for fear of retribution.

One information security expert warned that the site could now be vulnerable to hacks if overnight departures have depleted some engineering functions severely.

“Twitter, like any big website, is the target of lots of unsophisticated attackers and a few sophisticated ones,” said Steven Murdoch, a professor of security engineering at University College London. “Many security measures are preventative but some rely on skilled personnel monitoring for unusual behaviour, investigating these cases, and taking the appropriate steps to keep the attackers away from critical systems. Are these people still in their jobs?”

A US-based Twitter engineer who resigned on Thursday said he would have been “on-call constantly” had he stayed, dealing with complex systems he had no experience in because of the scale of cuts among engineering colleagues. Peter Clowes, a senior software engineer at Twitter, tweeted: “If I stayed I would have been on-call constantly with little support for an indeterminate amount of time on several additional complex systems I had no experience in.”

The news prompted an outpouring of concern among users on the platform, and prompted many users to link to their accounts on Instagram or Mastodon as an alternative.

US representative Alexandria Ocasio-Cortez tweeted she could be found on Instagram or on email.

She followed up with a tweet saying those employees who built the company deserved better. “Shout out to all the workers at Twitter. You all built a vital place for connection and deserved so much better,” she said.

Official government accounts also began providing means of being found elsewhere.

On Friday Ireland’s taoiseach, Micheál Martin, described Twitter’s behaviour as “unacceptable” and said that “employees of any company must be treated with respect and dignity.” Prior to Musk’s takeover, Twitter employed 500 people in Ireland.

Musk was tweeting through the drama. On Friday he posted memes mocking the expected demise of the platform and said Twitter usage was at an “all-time high” – something he blamed on media reporting about Twitter.

Earlier, Musk was meeting some top employees to try to convince them to stay, said one current employee and a recently departed employee who was in touch with Twitter colleagues.

In a private chat on Signal with about 50 Twitter staffers, almost 40 said they had decided to leave, according to the former employee.

And in a private Slack group for Twitter’s current and former employees, about 360 people joined a new channel titled “voluntary-layoff”, said a person with knowledge of the Slack group.

A separate poll on Blind asked staffers to estimate what percentage of people would leave Twitter based on their perception. More than half of respondents estimated at least 50% of employees would leave. Fortune Magazine reported that 75% of Twitter’s remaining 3,700 employees might have left on Thursday.

While it is unclear how many employees have chosen to stay, the numbers highlight the reluctance of some staffers to remain at a company where Musk has hastened to fire half its employees including top management, and is ruthlessly changing the culture to emphasise long hours and an intense pace.

Twitter, which has lost many of its communication team members, did not respond to a request for comment.

Blue hearts and salute emojis flooded the site and its internal chatrooms on Thursday, the second time in two weeks as Twitter employees said their goodbyes.

By 6pm EST, more than two dozen Twitter employees across the United States and Europe had announced their departures in public Twitter posts reviewed by Reuters, though each resignation could not be independently verified.

Early on Wednesday, Musk had emailed Twitter employees, saying: “Going forward, to build a breakthrough Twitter 2.0 and succeed in an increasingly competitive world, we will need to be extremely hardcore.”

The email asked staff to click “yes” if they wanted to stick around. Those who did not respond by 5pm eastern time on Thursday would be considered to have quit and given a severance package, the email said.

As the deadline approached, employees scrambled to figure out what to do.

One team within Twitter decided to take the leap together and leave the company, one employee who is leaving told Reuters.

In an apparent jab at Musk’s call for employees to be “hardcore”, the Twitter profile bios of several departing engineers on Thursday described themselves as “softcore engineers” or “ex-hardcore engineers”.

Reuters contributed to this report.