I probably won't use social networking sites unless they are useful even as a standalone page. I like google profiles and would enjoy extensions including reader integration, delicious-like page, and a good CMS site, etc. Allowing a public view version and private/friends version would push me to join the social network club.

Essentially, I would want it useful to me first and the social aspect as an addition. This can allow for slow take-off that would bypass some barriers to entry.