1. People use it for its personal value and network value is built as a side effect. (del.icio.us, youtube, google, flickr, amazon).
2. The idea is so compelling people directly contribute to the network value (reddit, digg, wikipedia).
3. You initially populate your site with so much data that its immediately useful.
What have I missed or gotten wrong?
Another way to start your social network is to gradually borrow someone else's. Youtube was immediately useful because links to Youtube videos could be emailed to friends and/or embedded in blogs and other social networks. The site played well with others, which eventually encouraged all blog readers, emailers, and MySpace users to start visiting Youtube directly.
You left out the super-secret invitation-only beta site method, where your first users don't mind the bare walls and the empty pages because they're so excited about being in on the ground floor of a New World-Changing Venture. After your first 10 users put in some data, you let the next 10 in. After you've got enough data that the site seems useful you start marketing in earnest.
A variation is the Craigslist method: use geography to your advantage. Start by building a Craigslist for your office building, then expand to your neighborhood, then your city, and eventually conquer the world. But this might just be a special case of point (1).