When it comes to a social world with people being fun rather than being depressed i would say:
1. Vanilla. Met alot of friends and saw lots of drama. It was so social that faction-racism existed. Wouldnt be surprising if an alliance player got beaten by horde players IRL because the only reason being hes playing an alliance toon.
2. TBC. Much like vanilla except only better because the game felt friendlier and more rewarding in my ways. I would say that TBC=Vanilla when it comes to being social.
3. WOTLK. This is where the game started getting depressed not because of the game itself, more like noone talked to eachother anymore. The guilds were still nice.
4. MOP. Still depressing but people helps out with eachother and speaks about how much they owned in the general chat. Atleast its a LITTLE bit better than WOTLK.
5. CATA. Worst expansion to date when it comes to being social. Felt like i played with people with severe depression. Even the GMs stopped messing around and joking with me.