Well, I'm new to Atheist Nexus, and I already like it a lot. It's neat to be on a social networking site with people who have share many of the same interests beliefs and experiences as myself, but there's still a lot of controversy and difference to keep things interesting. (I've already met a few people that I'm pissed at!) It's nice to not have to type in "religious beliefs" and worry about what people might think when they read it. It's nice to search for groups without feeling compelled to search out your fellow non-believers. There are no religious fanatics and no smug Christian groups or posts. While there is still plenty to disagree on, it's nice to join a social networking website where everyone here has at least one major similarity with you. It gives a nice sense of community, and a feeling of safety.
I had no trouble finding atheist groups on Myspace and Facebook, but it's an entirely different matter when you find an entire site full of them! I don't have to seek them out, they're all around me! I also don't have to deal with obnoxious religious chain-letter bulletins or Jesus-heavy Piece of Flair. I also don't have to be afraid that my atheist posts will offend anyone or cause them to think less of me. It's like breathing a huge sigh of relief.
We're supposed to be these amoral, selfish, lone-wolf type of people, right? Well, I think that the existence of a site seeking to unite us defies that stereotype pretty well.