Size of Social Network "Industry"

I was having a drink with a well known digital media executive this evening and the topic turned to, what else, social networks.  The question was how big of an industry is social networking?  I'm talking about the 'pure' social networks such as MySpace, Facebook, Bebo, Hi5, Tagged, LinkedIn, etc.  What would the aggregate revenues be of these sites?  Our guess was low single digit billions of dollars a year, if that, with the top few generating most of the revenues.

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[eyetag]

myspace had an estimated revenue of $550m last year. this year they expect $800m. if you add facebooks $150m and a few other smaller ones as german xing, which made about $30m in '07, your single digit guess seems pretty accurate. i would assume it's somewhere around $1bn, if you're looking to international sites only...