I remember back in high school when web games like Earth 2025 (games.swirve.com/earth/) were quite the rage and everybody was playing them. This lasted for a year or so until the novelty wore off. Some still play but the casual players are gone and the audience is a tiny fraction of what it once was.
I've noticed that my FB friends that play FB games tend to stop playing after a couple months. The games are mindless, cookie-cutter and repetitive for the most part.
<EDIT>I don't think WoW is a good comparison as there is clearly a huge depth between it and Zynga games. The experience is entirely different.
"Easy come, easy go" could apply to this market. I mean these games come out of no where to millions of players. These are not hardcore gamers that develop strong loyalty and build communities, but "casual" players that probably started playing because their friends are.
"Casual" gamers are notoriously fickle. Look at the Nintendo Wii, which is the most popular console but has the least average amount of titles bought per owner (which is where the real money is.)
In 2-3 years I would not be surprised that after all this over-exposure these x-ville games will be significantly less popular. These "games" have more in common with Tamagotchi and Beanie Babies than Wow or Call of Duty... </edit>