What about leaving the endless discussion about FB's destiny aside and just focus on creating a __new pattern of behavior or activity__ that appeals to a mass audience if you really want to build a large network on its own?

Examples of things that worked:

- Follow updates of celebrities

- Collect nice looking things on a website

- Make even the worst picture look good

- Be the best cat gif curator

Examples of things that didn't work:

- Facebook but for ten friends

- Facebook for Google Accounts

- Facebook with open data portability

Let Zuck worry about the hype cycle and start building.