I have never used Facebook - just not my thing at all. But I have a long history of meeting people online on various subject-focused websites and forums and I'm also in love with our SoberBook now. I think the theme-specific websites work for me because I usually also socialize, establish relationships, etc driven by my personal interests (shared with a group or another individual).
SR is definitely special... in my case I tend to think because 1. the utility factor (it's highly beneficial, not only just fun), and 2. the general culture and morals here.
I don't consider it as an addiction, though. It does not interfere with other areas of my life negatively, it's the opposite. But it's definitely a habit now. I am confident that if in the future I will want to shift my focus to other areas, I'll be able to hang out less on SR easily. But so far it's been adding only good things to my days and my recovery, so I stick with it.