They were a social network before social networks and always available IM and the gameplay provided a reason to be in the game (and therefore available to socialise with) even if you were the first/only one there.

I mean, they also had their own gameplay appeal, but I think the current player pool of WoW+FFXIV+ESO+GW is probably a more accurate reflection of how widespread the appeal of them as pure games is, while the crazy numbers of 00s WoW were about the social network aspect.