Two for me: MySpace and Melodramatic, only including ones where I'm active. Can't say I'm a big MySpace fan, but I'm certainly not a basher. Just because teenagers misuse the site, that doesn't mean it's lacking practicality. It's a great way for me to stay in contact with my girlfriend during the week. We're not phone people and we generally only talk on the phone at bedtime. It's also nice for keeping in touch with people without having to inconvenience myself and deal with certain formalities. I like that I can send a random message to someone to see how they're doing without having to look for their number (or get it from someone else), stop by their house, or show up at their job. I wanted to thank my girlfriend's sister for buying dinner the other night, and all I had to do was send a quick message. It helps me avoid the awkward small talk and I don't have to worry about waking her up or catching her when she's busy. I also don't seem nearly as creepy as I might if I had just randomly called her. So, the site is certainly useful for this sort of thing.
As for whether or not forums count as social networking...I think it depends on how you use them. On this forum, I don't socialize very much. On another one I used to frequent, I was there mostly to keep in touch with certain people. Although loose, I still think it counts. That's why sites like YouTube, 4chan, and even Neopets can count as well.