[Plugin: Sociable] Which sites to use

This is a general query from one who’s not across every social networking/bookmarking site out there.

Which sites should I be adding to my blog? I don’t want to include everything, or the more esoteric/obscure sites.

Is there a way the Pareto Principle can be applied… 20% of possible sites will apply to 80% of people? I’ve got about 90 sites listed in my Sociable Settings, so that would equal ~18 icons.