Since caving into the Facebook fever and opening an account, I've seen the following aphorism (paraphrased) come true:
Facebook is for friends I've met; Twitter is for friends I've yet to meet.
Next week, I'm gathering together folks at work to have lunch with former co-workers, some of whom I've really only met once or twice. Amazingly, we've been very in touch on Facebook.
In a couple of weeks i travel to Macworld and hope to run into some of the folks that I've been following and who are following me on Twitter.
These are not friendships I could have maintained without the Internet and without social networking sites. Who says the Internet stifles social interaction?