I've never played Farmville, so I probably shouldn't speculate, but I'm under the impression that it is at least ostensibly a social game where you invite people to help with or at least view your farm.

Yeah, but so is EVE and other games like it. I have never played EVE for any significant amount of time, but I did play WoW for awhile and there were tons of things that people would sink time into for purely social reasons. I imagine EVE is the same way. You don't just want a Titan because it is useful in the game, you want a Titan so you can have a Titan.

It's more of a social ponzi scheme. The fun part is the very-passive, easy progress. You just click on things. When you get people to participate in your farm, you're just saving yourself lots of time and clicks.