I was very active on Facebook for several months last summer, had nearly 1000 "friends" but then realized it wasn't where I wanted to spend my time online? One only has a limited amount of time in a day to spend online and I'd rather be here and other sites that are "helpful".

I deleted everyone except true friends and relatives and sent an email to those to tell them I wouldn't be on very often and that's it. I don't think you have to worry about the people asking "why", your "busy" is fine or tell them there are other sites you prefer to spend your time on besides purely social ones.