The "they'd never develop great features" argument is an interesting one, as I think most social networks can contribute most of their success to the network effect itself. Once that positive feedback loop gets churning, it's hard to turn it around.

Twitter's a neat counterexample here, I think. On the front-end, at least, the features released by Twitter in the last few years haven't left me awestruck. Yet it has become a seemingly self-sustaining space on the internet for public discourse.