I do a bit of social network R & D (still struggling to learn sociology, but, hey, I'm an engineer - their core classes were my electives) and you are right - it is an accepted fact that I take for granted that social applications are social networks. Just like cities are the projection on the ground of social relations, social apps (especially purely social apps like Facebook) are the projection onto cyberspace of underlying social networks.
Sociologists and other scientists love them because for the first time in history, people give them data for free, and subjects themselves make the efforts needed to compile and publish the data. Data is their for the taking! For exactly the same reasons, snoops too love them. NSA is doing so much research on social network analysis they could outpublish the entire academic community.