Facebook: I use it daily to keep in contact with overseas friends. I don't particularly like using FB as an intercessor in my friendships, and recently toyed with the idea of deleting it. I've realised that I will lose more than I'll gain.
Twitter: I have an account, but rarely tweeting. I use it more as a 'live rss' kind of thing.
Reddit: I use Reddit daily, but rarely post. I've been there for about 3 years, but my current account is only 6 months old. Unsubscribed to most default Reddits and actually finding Reddit pleasurable to use again!