It's not just the data, it's also what you do with it. I think a typical social network is a reasonably good fit to the relational model. But this is mostly because any given user cares only about his friends, and at most about the friends-of-friends. If you try to do something different, like computing eigenvector centrality of clusters of people, suddenly an RDBMS is inadequate and you want a graph database or something simpler like mapreduce.