Facebook denies that it's losing users, saying that it is pleased with growth.
Figures from Faceboook's monitoring site show that during May, the website lost six million users in the US and 100,000 in Britain.
But the social network site, which does not usually comment on third party statistics, has questioned how this figure was arrived at.
Other net measurement firms say they have seen growth over the same period.
Annenberg School digital future centre director Jeffrey Cole says Facebook is likely to become a victim of its own popularity.