I have searched deep inside my soul and come to these two reasons why I believe it will be a long time before I join a social network.
1) It's a time muncher: These days I find time is a precious commodity in my life and social networking (or the admin of it) would just eat away at time I never had before anyway. This blog and stumbleUpon take about all the time I care to spend networking/communicating on the net. (This blog was never even meant to be public anyway - It was originally designed as a type of personal jotter; Somewhere I could record ideas and URL's to interesting pages that I could reference in the future.)
2) This is the unspoken of, but everyone knows that they do it thing.. I believe a good portion (dare I say majority) of social network users find it a race to add as many 'friends' as possible to their profile. I suppose this is relative... some people will have many and some people will add friends slowly,.. but in the back of their minds they are all just dying to see their friends list grow.

So unless I needed to join one for a specific collaborative purpose, (or I won the lottery and could sit at home all day) I dont think I'll be on your friends list anytime soon ;)