Casinos offer a variety of different games, which have different appeal to different types of their clientele. There are lottery type games, in which there is a faint hope of a extraordinary windfall, which appeal to people who want money without working for it (i.e. people in debt). There are games in which there is a large degree of control and which the winner emerges because of his/her psychological prowess which appeals to the serious gamer (i.e. professional poker players). And then there are the run of the mill ring-ding-ding games, which appeal both to the idea of winning money, the suspense of the spinning wheel, and various other attention keeping mechanisms.
Social games have the ability to emulate virtually all of these. Although ostensibly buy-in is limited to virtual money that can never be removed from the game, there is nothing stopping large markets to grow around virtual items that can be traded (as happened with Ultima, EQ, SL, Eve, etc.).
The clutch here is that the "fun" of the experience in both cases largely revolves around ideas of advancement, social enhancement of the idea of advancement, and (potentially) some real world tie-in.
Actually, in many respects the social game is far more advanced than the casino already, insofar as it offers the idea of advancement which is potentially not tied to money (i.e. leveling up), but which can also be tied to money if people want it to be.
The problem of this right now is the fact that virtual currency cannot be liberated, but as soon as that problem is solved the casino will be an obsolete institution, soon to be extinct.