It doesn't matter. Use the one you understand better. Both types of databases have been used to build social networks with many millions of users. Chances are

It doesn't matter. Use the one you understand better. Both types of databases have been used to build social networks with many millions of users. Chances are ~0% that you'll ever have enough users to care which one you chose, and if you get to that point, you'll be able to raise money to switch. Your main problem building a social network is acquiring usere, not technology.