Opening admission: I'm coerced into playing Farmville and Fronterville by my Mother and a couple of friends who want me to send them gifts and occasionally do crap on their farms. Also, I willingly play the D&D Adventures FB game, and I've tried the 'just barely a game' type stuff like Mafia Wars.
To my knowledge, all the Facebook games are free. Lets assume that Farmville was an 'indie' game. If the game provides you with some level of enjoyment, how is dropping $15 once off for some extra game content any different from paying $15 for some indie game that you might play for a week or two on and off before finishing it or being done with it. I suppose once you start to spend a substantial amount of money it's a different issue, but then that's not specific to Facebook games. It does make me wonder if anyone I know has spent money on these games, I must admit.
Is the fact that the goods are 'virtual' such an issue? This will start an argument, but how tangible are any of the mp3s that you purchase from say, iTunes, or books via Kindle? Yes, it's an mp3 or a glorified text file, that provides entertainment, or whatever you want to define it as, but it's still entertainment in virtual form. Really, how different is it to purchasing goods for some subjectively entertaining virtual farm; at the end of the day is it still not simply entertainment in an intangible form? How is this not just a digital way of buying extra dolls for a dollhouse or some other real world to virtual comparison that might have not implied that I own dolls?
Each to their own, seriously.
Also, you can walk in and touch swampland in Florida. That's way more effort than dragging some fences and cows into a virtual lot on my PC. It's a totally different market ;)