Well 37signals is not a social network. They are however probably the only company that is profitable.
The market size of social networks are potentially limitless but even though it's fairly easy to make revenue it's very hard to be profitable.
First and foremost because most social networks are ecosystems and start out as free.
Second of all because it's very hard to get people to pay to work which is potentially what they do when they join a social network.
But you can work with the following parameters to determine potential market.
Google ads, Premium Ads (ala those on Smashing Magazine)
Well.com as I have been a member of for 10 years now is a subscription based discussion forum. They haven't changed for years yet it's the place I know with some of the smartest people on the planet. You can course also calculate based on the "freemium" model
Free to play, pay for sway.
This is very popular with habbohotel.com and with a lot of casual games.
If you want to calculate the market size I would suggest you simply multiply number of users with the unit prizes for the above mentioned possible models.