Techcrunch has a post that details how Facebook Users Up 89% Over Last Year; Demographic Shift with ".. an 89% increase in unique visitors to the site at 26,649,000 uniques, with a 143% increase in page views at 15.8 billion. The site’s stickiness has increased and then tapered off at about 190 minutes per average user."

I've gotten a couple of requests for friends on Facebook, and I have a profile there - but don't use it that often.  I found the data in this post useful though.