Like Howard and Atom, I've not held an account on any of these sites, nor do I have any kind of personal desire to do so. However, as DaveMMR said, it could be an excellent networking tool. Which is something I sorely lack. I know very few people on a personal level, which can hurt in a tough job market. Fortunately I still have mine, so that doesn't currently matter.
If I ever do one of these sites, it'll be purely for networking reasons. I have better or more fun things to do than constantly update it with drivel and 80,000 meaningless pictures. I don't have much interesting to say. Then again, from the posts I've seen other people put, nobody does.