The number of unique UK visitors to declined by 300,000 to 6.5m in May against the previous month, while gained 527,000 visitors over the same period, according to Nielsen//NetRatings.
Facebook attracted 3.2m unique UK visitors in the month and is expected to continue to grow in popularity as the site benefits from acres of free press coverage and word-of-mouth endorsement.
Bebo, the third-biggest social networking site and particularly popular with teenagers, has experienced steady growth since April, with an audience of 4m in May.
The trend is mirrored across the Atlantic, with MySpace suffering at the hands of a resurgent FaceBook in the US.
MySpace's unique US visitors declined by 450,000 to 56.6m in May, while Facebook's US audience has grown by a similar margin to 14.2m.
In the UK, Facebook has a greater reach that its US version, with 10% of the UK population logging onto the site, compared to 9% in the US.
But MySpace is more dominant than Facebook in the US, with an audience four times the size of its rival. In the UK, MySpace is only twice as big as Facebook.
Alex Burmaster, European internet analyst at Nielsen//NetRatings, said: "MySpace is, by far, still the most popular social network. However, if last month's growth rates were to remain consistent, both Bebo and Facebook would catch MySpace in September this year.
"Meanwhile, MySpace does trail the other two when it comes to user engagement. Bebo leads the way, with its average visitor spending just over an hour and a half on the site -- almost 10 more minutes than the average 'Facebooker' and almost a whole hour more than the average 'MySpacer'."
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