marketingmagic -- I just posted some numbers a few hours ago:
"However, for those that want real numbers, we've had about 5,000 visitors this month from our Facebook page and about 2,000 from Twitter. Neither one has huge numbers of followers/fans, but the ones we have are legit. No games, no shortcuts -- just steady growth."
Certainly not huge numbers, and overall a fairly small percentage of our total traffic, but it's certainly something we're keeping an eye on as social services continue to grow.