Thursday's $850m purchase of Bebo by AOL provided a rare data point for industry-watchers who are trying to determine how much fast-growing social networks are worth.

The question has swirled since Rupert Murdoch's News Corp bought MySpace for $580m in 2005. None of the large social networks disclose sales or profit figures, and their buyers' differing strategic priorities make direct comparisons of the amount they are willing to pay for social networks difficult.

Though staggering at the time, MySpace's price tag was rendered quaint after Facebook, its biggest rival, drew an eye-popping $15bn valuation in a recent investment round led by Microsoft.

However, a look at recent visitor traffic suggests that Facebook remains an anomoly, and that AOL may have snapped up Bebo at a relative bargain. Bebo ranked ninth worldwide among social websites in terms of overall user traffic as of January, with 22.4m unique visitors, according to ComScore, the web traffic research group.

Crunching the numbers reveals that AOL paid about $37.95 for each Bebo visitor. A similar analysis shows that Microsoft paid about $149 per visitor for its 1.6 per cent stake in Facebook.

Some recent reports have suggested that MySpace could be worth between $6bn-$10bn today - implying a price per user of $54 to $91.

Additional reporting by Andrew Edgecliffe-Johnson in London