I've been on MOG for about six months and I quite enjoy it. I would say it's more of an online community than a social networking site. Unlike sites like MySpace and Facebook, keeping up with friends (or, let's be honest, looking for sex) is not the reason for its existence. Instead, it's a place for music geeks to blog about and discuss new music, news, concert and album reviews, and anything and everything music-related.
It also has a feature, much like last.fm, where it tracks what you listen to, generates weekly and monthly charts of top artists, albums, etc., and makes recommendations of stuff to listen to. And they've just added butt-loads of new features to help you discover new music.
I think it's a great site, and highly recommend checking it out.
