This article seems to suggest that Zynga (the creator of Farmville) is "a company that has made a name for itself by acquiring or emulating many of the most successful games on Facebook and MySpace." http://www.insidesocialgames.com/2009/07/15/with-farmville-zynga-joins-the-facebook-farming-fray/ Agree?
Zynga was pulled into court for making Mafia Wars, by the original makers of Mob Wars. Mob Wars is an inferior and much lesser-known game to Mafia Wars, even though Mob Wars came first (which is sad). Zynga just has an amazing dev team and knows how to market. That being said, Zynga is a total copy cat...they just cross-promote the hell out of their games, which makes them so popular. At this point, they're crapping gold...no matter what new game they make, it becomes an instant hit.
Yes, so in a way to succeed, for the technical part you can just copy and make a better improved version- (it does not have to be a unique game) and then spend loads of $$ on marketing/ promotion