overwhelmingly the value of FB and other networks comes from the fact that so many people are on them, and those people all create content for free.
No it isn't, google isn't about people's content, Google is about letting others find "their" content if you want to boil it down to the bare basics, facebook cannot do anything with people reposting memes grabbed from elsewhere on the net its a big data network always was, it's why anyone advertising on the site can target demographics, google just have more integrated ways of gathering big data now.
It's not like facebook and google have a vast army of workers whose wages need to be paid now is it, the overheads are low because we do all the work for free, we interact with the interfaces and train the AI so that they can build complex and detailed profiles of as they monetize our data.
I work for google, I earn a very large salary as do most in my department, I suggest you look up the salaries in Silicon valley.
I think you have shown yourself up as not knowing very much to be honest (in the nicest possible way), just talking about facebook here, but where do you think all their data is stored?
Hardware costs, replication, sys admins all cost a huge amount of money, there is an awful lot of stuff behind the free social network sites that adverts alone cannot cover.