Facebook lost 100,000 British users last month as concerns mounted over privacy.

Fearing for their privacy, or perhaps just bored with the site – in a symptom that has been dubbed ‘Facebook fatigue’ – the Brits are said to have deactivated their accounts, along with six million who also logged off for good in the U.S.

Earlier this year, executives announced that the number of Facebook accounts held in the UK had reached 30million, accounting for about half the population.

The milestone was an increase of four million from last July and represented the highest saturation of any country in Europe

But times change and, worldwide, Facebook’s rate of growth has slowed for a second month in a row.

As it aims to reach its goal of one billion active users, it is having to rely on developing countries to boost its numbers.

The figures suggest Facebook may have reached ‘saturation’ point.

There is even speculation on blogs that, as is feared for its failing rival MySpace, the website could one day “sputter into oblivion”.

Facebook, with 600 million users, has been rocked by privacy scares of late, especially over its new facial recognition service.

The U.S. Electronic Privacy Information Centre and three other advocacy groups today filed a complaint asking the U.S. Federal Trade Commission to force Facebook to end plans for the service.

Their calls were backed by Ed Markey, the U.S. Representative for Massachusetts’s 7th congressional district. He wants the site investigated.

“When it comes to users’ privacy, Facebook’s policy should be: ‘Ask for permission, don’t assume it,'” said Markey, co-chairman of the bi-partisan Congressional Privacy Caucus, in a statement today.

“Rather than facial recognition, there should be a Facebook recognition that changing privacy settings without permission is wrong. I encourage the FTC to probe this issue and will continue to closely monitor this issue.”

In the U.S, Facebook user numbers dropped from 155.2 million to 149.4 million throughout May.

In Canada there was also a fall, of about 1.5 million users, while in Russia and Norway numbers also fell by more than 100,000 users.

It’s not all bad news for the site, however. Worldwide, Facebook is still expanding and has around 600 million users, thanks to strong growth in countries such as Mexico and Brazil.

According to Eric Eldon, of the website Inside Facebook, which obtained the figures through analysis of the company’s advertising tools, there is a point at which the site can no longer grow, once it has established itself in a country.

“By the time Facebook reaches around 50 per cent of the total population in a given country, growth generally slows to a halt,” he explained.

Internet psychologist Graham Jones predicted that Facebook users would suffer the same kind of fatigue that comes whenever men and women get bored with trying anything that is new.

He said: “People get terribly excited about something new and after a while the novelty wears off.

“Even if it is a new TV series everybody thinks it is fantastic at the beginning and things tail off.

“In all aspects of our lives we are addicted to novelty, so Facebook should be the same.

“The reason it is so compelling is that it is the first big website that allows two-way communication between people.

“Humans are social beings and up until about five years ago we did not have a website for direct communication in this way.”

Facebook under fire over facial recognition system

Facial recognition across Facebook has raised the ire of privacy advocates and some users.

The European Union’s data protection regulators were the first to take on the issue, immediately announcing plans to launch an investigation. Authorities in the U.K. and Ireland then disclosed that each is considering their own probe.

Facebook last week said it’s already trying to answer questions from EU regulators, and now may face queries from U.S. officials.