My opinion?
There's nothing wrong with any store restricting which web sites people can visit on the demo computers. I've been to plenty of stores where the machines didn't even have Internet access at all. People still tinker around on them enough to figure out the approximate speed they perform at, etc. and they still buy them afterwards.
Apple tries to raise the bar by encouraging full use of their systems on the net -- but if a certain site is just drawing the wrong crowds in (people with no intention of buying at all, etc.), then it makes sense to block those sites.
That said though, I think Facebook is a really useful tool for those of us in our 30's or 40's. I always got the impression that myspace was the site taken over by mostly teenagers competing to see who could collect up the most "friends", and make the "hippest, coolest" backgrounds on their pages. By contrast, Facebook tends to look much "cleaner" and more professional on the typical user profile page one visits.
Sure, it can be a bit addicting - but as a late 30-something myself, I know how many of my old friends have drifted away over the years. Facebook was an incredible tool for locating most of them and linking everyone back together again. (Friends of yours see other friends you added who they also know, and then they add each other too, re-connecting the whole "chain" of friendship.) The "status" part? Sure, people update it with boring/dumb stuff most of the time. But as much as anything, it's a way to easily see that someone you know "checked in" recently - and over time, it really does give you a better sense of what your good friends do in a typical day.
I think it's a really good move but I am not so sure about it being a) primarily teens and b) a limited shelf-life with users.
In our department there are at least three people who are always all over Facebook; all are women in the their mid to late thirties and all have been "Facebook, Facebook, Facebook" for well over a year now. Not having Facebook is considered really odd and you are always asked why you haven't and why you don't think it's as wonderful as they do. Personally, I think they answered the question themselves when one of them changed her status to "eating a baked potato" and then, at the end of lunch, changed it to "thinking about the potato I just ate"... it's really earth-shattering stuff, people!