I was a big fan of Glitch (created by some people from Flickr I believe) before it abruptly ended this past Fall. It had a really cute "melancholy" feel, and it encouraged interaction and cooperation between people in real-time play, and in that aspect it was very "pro-social". It had lots of mini-goals and artifacts and stuff that were fun to explore, and the underlying narrative was fun too.
I'm still looking around for something that I can get into at that level, with not so much luck so far.
I am not familiar with Glitch, but they are trying to bring back the old Atari game M.U.L.E and a friend of mine sunk big bucks preparing for a Kickstarter funding, but it failed although they haven't given up. Here is a link to the original M.U.L.E. site where they talk about it. You might like it. YouTube Presentation on M.U.L.E.