I had created a Facebook account several months ago, but had no friends added because I really wasn't in to commenting. When I started 23 Things I decided to go back on and add some friends. I was amazed at how quickly my wall grew.
I had some concerns when I had added a political group from Iran that protested the election and heard that the Iranian government had begun monitoring Facebook and Twitter. I was afraid that my husband's nieces and nephews who live there could have problems, so I removed the group. I think I just got paranoid-- but better safe than sorry. I still monitor Twitter for news being sent out of Iran.
As for using this in the classroom, I'm not sure yet how I could use it. Most students are MySpace users and they would have no problems using Facebook. One thought would be to post homework help,reminders,etc, but this could also be done on a classroom blog. The jury is still out on its classroom use. Maybe others will have better suggestions.