With the introduction of server transfers in TBC, a lot of the sociability of the game went through the window.
All the server reputation you had, either good or bad, slowly was vanishing after its implementation. I believe it was the first dent at the social aspect of the game, as it stimulated anonymity and toxic behaviour within the community. You could be a douchebag, steal Illidan's Warglaives and move on, literally, to a new server.
LFD and faction change services, implemented during WotLK, made the situation even worse.
Cataclysm just brought Sociability to a new low, with LFR.
I am expecting Blizzard will introduce LFD with NPC's someday, like they did with Proving Grounds.
No queue times, no hassles, but also no social experience.