This history of FarmVille implies it was better game mechanics than Farm Town, and Zynga's installed base, that lead to FarmVille's success: http://www.allfacebook.com/2010/01/farmville-history/.
I'd also hypothesize that Zynga grew because they:
* Had a working revenue model, so they could pay for advertising without burning cash. And being able to advertise in the place where someone plays is helpful for conversions.
* Built on a growing platform (i.e., Facebook)
This history of FarmVille implies it was better game mechanics than Farm Town, and Zynga's installed base, that lead to FarmVille's success: http://www.allfacebook.com/2010/01/farmville-history/.
I'd also hypothesize that Zynga grew because they:
* Had a working revenue model, so they could pay for advertising without burning cash. And being able to advertise in the place where someone plays is helpful for conversions.
* Built on a growing platform (i.e., Facebook)