It's not just for the sake of it that I like these sites; I've found that as an illustrator, they're very useful tools: You can let people know what work you're doing, where you're going to be selling stuff, etc. I've gotten some interesting commissions through these sites and a lot more interest in my work. Setting a page up purely for my "business" has been great too as it's a very nifty thing to be able to give people Facebook details at comic book conventions. FB seems to be an acceptable form of professional networking now which I quite like as it's a very accessible system. However, like most web-based promotional tools, the more you put into it, the more you get out of it.
How does everyone else find social networking? Does it work for you? Do you do it just because you feel left out otherwise? What are the perils of using social networking in a professional capacity? Gimme your opinions! I would like to do a follow-up post to this so the more the merrier! :P