that's pretty much how the f2p subgenre of the mmo genre works. they get your foot in the door with the free start up cost and then reel you in witht he addictive grind and then pretty much force you buy stuff to get any further, adn there are many people who are willing to pay far more than the standard p2p mmo fee of $15 a month on these kinds of games in very short period of time.
Zynga definitely has some great casual games. They can be fun and extremely addictive. Zynga is also a bunch of greedy ********ers and you can easily end up spending much more than you would for a well developed traditional game that spent months or years in development by much more talented people. I usually play their games until it gets to the point where you are forced to spend money to progress reasonably in the game and then I drop them.
and in teh case of zynga games they have fb users spamming their friends with invite requests daily to add new players to the potential customer base which far surpasses the marketting ability of absolutely any other game company in terms of reach. i know i've had to block every new fb game request on fb or my friends keep sending them.