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.