Are you kidding me? Do you really think the folks who developed myspace, or facebook (et. al.) did it completely out of charity? Furthermore, if they did, they certainly have an odd way of taking usable space away from the user of said service, and filling it up with those bastard adverts.
Personally, I don't think a social network business is anything more than maybe one or two really bright people, shadowed by an army of managers, directors and bean counters, who are, in all actuality, too lazy to hire any programmers to provide interesting content.
The bright people develop the code to allow users to "create" web pages on that site, that the company later injects ads, in hopes that some idiot will accidently click on, for another 5 cents of revenue.
In short, it's a scam at multiple levels, and any "worth" the company may be quoted as having is only worth the depreciated amount of their servers, after you deduct any monthly operating costs.
Remember Angelfire, Geocities and even AOL? None are even interesting blips on the consumer radar anymore...