This difference means more than just the ability to chat with your fellow players and send gifts directly to their chair, though both are nice features. It means that Zynga Poker is a true battle of competitive multiplayer strategy, rather than merely a story of inexorable progress. The basics of which hands win and what hold cards are good can be learned in no time, but the vagaries of betting strategy and bluffing can take a lifetime to master, and vary depending on the personalities of the real people you're playing against. In poker, you're playing the other players as much as you're playing the cards.
So it's not surprising that poker is one of the most popular genres on Facebook. Zynga's version is the most popular of those on the network, helped along by cross-promotion from Zynga's other games, but also by an excellent interface. Cards are large and easy to read, and the game provides helpful sound effects to help you follow the betting action. The small bell that sounds when its your turn to act is especially nice, making it easy to follow the game even when you're multitasking. The interface is also helped by handy checkbox buttons that let you check, fold or call any bet before it's your turn to act, so you don't have to constantly be on the watch for your chance to act.
The only real problem with Zynga Poker has to do with the flood of virtual money that's constantly being fed into the system. Zynga Poker throws chips bonus at players for practically everything. You get bonus chips for signing in daily. You get bonus chips for winning certain hands and accomplishing other achievements. You can get chips as gifts from your poker playing buddies and for completing team challenges. You can even get chips just for clicking a randomly appearing button that says "get 2000 free chips" that just pops up. Getting chips in Zynga poker isn't a matter of poker skill, it's a matter of patience.
The net effect of this glut of chips is that people don't play logically. Since players aren't playing for real money (for the most part -- you can buy chips with real cash if you really want) and since it's nearly impossible to totally run out of chips, players bet much more liberally than they would in real life. In a full nine-person table, it's almost impossible to bluff out the opposition, because there will usually be someone who decides to call your bet just for fun. And a good portion of the time, that someone will luck into a good hand that he had no right even playing, just because he can. This problem is lessened in tournaments, where players can only lose a set number of chips before leaving the table, but it's still a problem.
Even with this major flaw, the social elements, smooth interface and timeless gameplay showcased in Zynga Poker make it perhaps the perfect shpwcase for what's special about Facebook gaming.