Are you talking about personal usefulness or usefulness for IM.
For personal usefulness, I think it is pretty useless. You just get to read all the complaints and stupid status updates of everyone you know. Some will say they had a good day and some will say they had a bad day. I have even seen people updating that they took a big crap and that they feel better! Useless. I think facebook is very popular because people like to keep track of what others are doing. They are all jealous and curious about how people they know progress in life and then compare that to their own progress.
For IM, it can be pretty useful. If you create a story that goes viral and link it to a CPA offer, you can make big bucks. Upgrade to JR.VIp to learn more about FB marketing. I think they have a lot of posts about that there. I was never interested in it though.