Ask HN: How to drive user aquisition for my new social networking application?

I'm developing a new social network: one that could change the way people communicate for the better. Good applications launch every year but that's never enough.

What are your suggestions on how to drive users to join, invite friends & come back often?

IDEAS Launch at one university or large network only. Using an "invitation only" waiting list strategy. Forcing users to invite a few friends at sign up.

Any more?