Let's be honest here, most MMOs actually promote solo play and selfish behavior. Most games will force you to party play eventually but it's mostly a requirement for your OWN advancement. There's no sense of community because the only reason people party or run dungeons together is so they can get their OWN set of epic gear. I don't care about your raiding requirements, I care about how many tokens I need before I can get that orange/exotic/epic/amazing sword of doom, if you aren't useful to me then you might as well be gone.
A truly social MMO would be one where players are forced to work towards a common goal that benefits ALL of them. Wurm Online or H&H, sandbox games where you build towns/cities and need to defend them actually promote this. You aren't required to connect at certain hours because there's no need to party, you can venture into the nearby woods, collect some wood, get some food and bring it to town, you are helping your fellow players without playing with them. This sort of behavior creates a close community.