From eMarketer, via VentureBeat.
In summary still rising fast, but not quite as quickly as they had thought previously. $920m last year.
And Myspace is the premier destination, although Facebook is taking market share.
Note also that widgets only accounted for $15m last year, and will still be small this year.
Finally, as Venturebeat points out social networks make money via ecommerce and other areas in addition to advertising and therefore these numbers do not equate to total revenues.