Density density density. You need to identify a core set of densely connected, ~1,000 people to get your social network started.

MySpace had the LA music scene. Facebook had Harvard. LinkedIn had the Silicon Valley hotentots, thanks to Reid's reputation and network. Quora has a similar group of highly active, densely connected users from Silicon Valley.

So you need to think about the ideal community for your network and find a way to get 100 to 1,000 of them on your site ALL AT ONCE, i.e., within a few days of each other.

The onboarding experience is absolutely crucial, too. There's a paper out there by Facebook's data team and some CMU researchers about what early experiences (on Facebook) are the strongest predictors of long-term engagement.

Not surprisingly most of them involve some sort of interaction with a friend. I don't have a link, but you should find it and read it.