I use different social networks in different ways. If I don’t add you on Facebook, this explanation might help.
I don’t want to offend anybody, but I like to have a well-defined target group for each network I’m a member of. This cuts down on the decision-related stress I face from using these systems.
Facebook. I reserve this for family, old friends and people who live locally to me. I do not add any professional or business contacts here.
LinkedIn. I accept connection requests for anybody that I’ve met, even if it’s just by phone. Sometimes I accept requests as a “I’d like to meet you” signal, but rarely. I mostly ignore messages from people I don’t know.
Foursquare. I only accept connection requests from friends who would understand if I said “sorry, I can’t have dinner, I’m too tired.” Fortunately, nobody really wants to add me on Foursquare, so the decision rarely needs to be made!
As in regular life, I tend to prefer to meet new people online through introductions from existing friends and contacts.