There is the question of use-value vs. exchange-value, and you could argue that social networks, as with other electronic communications systems (say, Craigslist) serve mostly to facilitate activities which wouldn't otherwise have occurred, and/or leave money in the pockets of those who make use of the services (where you'd previously have had to use newspaper classified ads, or staple flyers to telephone poles, or stand on a soapbox in the town square, or go out and meet your friends in person).

Most of the social networks have had fairly modest revenues. Let's see.... Facebook are claiming $2.5 billion for the three months ending 2013-12-31. That's actually not too bad. Google's $16.8 billion. Most of that's advertising, and if you think of it that way, Google, Facebook, and the other social networks are skimming off of the total advertising market (not insignificant).

A horrible infographic showing Google's ad spend breakdown: http://www.wordstream.com/articles/google-earnings

(When did the world forget how to create bar plots?)

Here we go: http://i.imgur.com/tTVUkwL.png

Roughly 10% of that is finance. The rest various consumer categories. That's the economic impact of social networking, roughly.