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Facebook CEO Mark Zuckerberg speaks during a media event at Facebok headquarters on April 7, 2011 in Palo Alto, California. - 

HOBSON: Facebook just lost a few faces. Six million users in the U.S. ditched their Facebook accounts last month. And the number of people using the site during the month of May also fell in Canada, Russia and the U.K. That's according to new data from a company called Inside Facebook.

The BBC's Rebecca Singer has more.

REBECCA SINGER: Facebook is still racking up users in new markets, like Mexico and Brazil. But according to this data, it seems that once half of a country's population is logged into Facebook, the social networking site stops growing.

Chris Green is a technology industry analyst at Davies Murphy Group. He says that makes up most of the core market.

CHRIS GREEN: If you think about it around about half the population, a little bit more a little bit less depending on where you look, will comprise of those between about 16 and 60. Anything outside of that, or those really too young to use the service or appreciate and understand it.

But don't write off further expansion in the countries where it was first successful.

Green says although Facebook isn't actively reaching out to older users yet, the over 60s are increasingly being drawn in by the big advertisers, who are intentionally trying to bring older users onto the site because they have so much spare cash.

In London, I'm the BBC's Rebecca Singer for Marketplace.