I think that they are pretty useful especially if you want to call attention to something that you like. In fact, when I start planning my wedding, I intend to set up a FB and Twitter account so that I can send the family and friends on my FB things that I am looking at and that I can get their opinion on.
Some are on Twitter as well, and although I hate it, I can see the usefulness in it, especially if I want to make a mental note of something for later. I don't exactly have the best memory.