Why trust logging on when you can talk to friends face to face?

Facebook was quick to dismiss the news last week that it lost over 100,000 UK users in May as nothing more than a blip. Yet when it came to explaining what caused the mystery drop, the company appeared to be as bemused as everyone else.

So why are people logging off Facebook for good?

Trust is clearly a big reason. In May 2010, social media website Mashable ran a poll in which over 30 per cent of those surveyed said they were quitting Facebook because "I don't trust it with my personal information". Since then, confidence in Facebook's approach to privacy has continued to tank, not least because of the depiction of founder Mark Zuckerberg in the film The Social Network (pictured above) as a ruthlessly self-interested megalomaniac. Facebook's recent decision to introduce facial recognition for tagging photos is the latest issue.

"Privacy fears have always been around Facebook, but they are magnifying," said Giles Lury, the director of branding at The Value Engineers. "For consumers, face recognition is scary. In Britain, we've always been slightly reticent of Big Brother watching us, particularly on a digital level. Just think about the reaction to ID cards." When Facebook appears heavy-handed with privacy, he argues, users leave.

Another potential cause for the recent flood of departures may, ironically, come from Facebook's own success. In the developed countries where numbers also fell drastically last month – the US, Canada and Norway – more than 50 per cent of the online population now has a Facebook account. While users may initially have joined Facebook simply to socialise with friends, they now face the unappealing prospect of having to interact with parents, bosses and co-workers.

"People are becoming aware of the limitations of Facebook's semi-public, 'all of your friends under one roof' system," said Dr Bernie Hogan of the Oxford Internet Institute. "As soon as mum joins Facebook, people have to make sure Facebook is mum-safe. If a teacher becomes friends with a student on Facebook, they have to make sure the content is appropriate for their role as a teacher. What that does is water down one's capacity to fully express oneself in this space."

The result, according to Dr Hogan, is that people are going to different sites for their specific online needs. A person who wants to network professionally, for example, will use LinkedIn, while someone who would just like to share their thoughts might get tweeting. For these people, a one-stop social network like Facebook is redundant.

Recent Facebook quitters cite more mundane motivations for leaving: it took too much time; it got a bit boring; I wanted to concentrate on exams.

For Katie Clark, 27, a Londoner who quit in 2009, the whole thing just seemed "a bit voyeuristic. It's such a waste of time," she said, "scanning through all these virtual friends who you're never going to see again, people taking posy pictures of themselves so they can look good on Facebook. I just thought it was a bit sad really."

Clark's frustration with the artificiality of Facebook's social life is becoming more and more common, according to psychotherapist Sheri Jackson. "Facebook can contribute towards depression and anxiety," she said. "You may feel you are not as popular as other people because everyone's lives are on full display – everything's open for comparison."

"A lot of the reasons I hear for people leaving Facebook are because of its intrusive nature," she said. Ultimately, it seems many users just grow tired of keeping up a public profile and quit, knowing their real friends will always remain in contact. "People think that rather than going through the whole hassle of checking all their posts," Jackson said, "they'd rather stick to old-fashioned one-to-one contact."