Facebook’s troubles entered a fourth day with a rising chorus of people – including the co-founder of WhatsApp – joining the #DeleteFacebook movement as the Federal Trade Commission was reported to be investigating the company’s handling of personal data.

Momentum gathered behind the #DeleteFacebook campaign, with several media outlets publishing guides to permanently deleting your Facebook account. One surprising voice to emerge was that of Brian Acton, the co-founder of WhatsApp, which was bought by Facebook for $19bn in 2014.

Acton, who left WhatsApp in late 2017, posted to Twitter: “It is time. #deletefacebook.”

The backlash has spooked investors, leading shares in the company to fall by more than 9% over the last two days – wiping almost $50bn off the valuation of the company.

Meanwhile, in the United States, the FTC will examine whether the social networking site violated a 2011 agreement with the agency over data privacy, after reports that a firm called Global Science Research harvested information relating to 50 million Facebook profiles and provided the data to Cambridge Analytica.

In the UK, MPs summoned Facebook’s chief executive, Mark Zuckerberg, to give evidence to a select committee investigating fake news.

Damian Collins, the chair of the digital, culture, media and sport committee, accused the company of misleading the committee over how third parties acquired and held user data from its site and whether it had been taken without consent.

“Your officials’ answers have consistently understated this risk and have been misleading to the committee,” he wrote. “It is now time to hear from a senior Facebook executive with the sufficient authority to give an accurate account of this catastrophic failure of process.”

Neither Mark Zuckerberg nor the chief operating officer, Sheryl Sandberg, have made any public statement about the scandal, attracting further criticism over a lack of leadership. Nor were they present at an all-hands meeting for employees at Menlo Park, California, earlier in the day, led by the company’s deputy general counsel, Paul Grewal.

Zuck did, however, have time to “like” a picture posted by the Spotify CEO, Daniel Ek, and a status update by an early investor Don Graham, the Huffington Post reported.

“Mark, Sheryl and their teams are working around the clock to get all the facts and take the appropriate action moving forward, because they understand the seriousness of this issue,” said a Facebook spokeswoman. “The entire company is outraged we were deceived. We are committed to vigorously enforcing our policies to protect people’s information and will take whatever steps are required to see that this happens.”

• Contact the author: olivia.solon@theguardian.com