One thing Blizzard have been bad at for a while now is keeping a good community. When you base things on a facebook style, total lack of privacy (profile viewing in D3, Armoury in WoW), talk down on the customer base as a matter of course, target a younger demographic that is often quick to talk crap, remove as much complexity from the games as possible (MoP = WoW for kiddies, D3 removing skill trees, skill points, resist caps, etc, etc), well the collection of all of these things is a community that is not too healthy.
Then Diablo 3 was not a AAA title. It lacked a lot. It doesn't look "amazing" like an AAA title should. It had nothing to do but endlessly grind acts over and over. No making character builds destroys the game. So setting the bar that low, I can imagine that TL2 is roughly equal in size to D3. Now TL2 wins because of community content and offline play, which help foster good community and goodwill.
Now when I play Path of Exile, in the end game stuff, I see people talking about different builds they want to try, how certain skill mechanics work, talking about maps they are running or not finding :). The chat is always busy. When I play D3 the chat is empty, except for the odd gold spammer and people with wts [junk] messages. I have not played TL2 but I am confident when Path of Exile finally gets it's full release it will blow D3 and TL2 away, it is just that good.