The OP is quite biased (yes, I know you announced it
)
The main reason I am on a lot of these sites (Facebook, myspace, XuQa, etc.) is because of common courtesy. I look at other people's profiles, so why shouldn't they be allowed to look at mine?
Also, sites like these allow people who won't see each other ever again (literally) to stay in contact with one another. I've reinitiated contact with several of my high school friends I would have otherwise never talked to again. That's a good thing.
Also, it's helpful in determining vital information, such as "how many IT majors there are at my campus," "does girl X have a boyfriend," and "how many people share similar interests as me?"
A lot of my college professors are on Facebook at my university. It's not some fad or social trend that will wither away, it's a new feature the Internet allowed that's not going anywhere for a long time.
Most people that bash it haven't used it or are clueless to how it works or how fun it can be. Of course you admitted that in the OP
Give it a try. It's fun and you might meet new people and/or keep in touch with old friends. It's a great way to see what your colleagues are doing nowadays.