Obviously it's not about "exploiting their friends". You can block Farmville or Mafia Wars if you don't want to see updates about them. There's a cooperative element that a lot of people enjoy there; they enjoy bartering with friends for items and sharing the wealth when they have it.

Farmville is The Sims: Farm Life pared down to work in Flash on old computers. It is popular for the same reasons The Sims was popular and it appeals to the same demographic, but it is even more popular because Facebook has become a vector to involve _real-life_ friends and to allow people to get sucked into the game and play for a few minutes at a time.

It's a pretty simple formula, nothing really ground-breaking there. Some classic RPG elements + interior decoration + cooperation with friends + negligible chunks of time commitment to accomplish things = high appeal to certain groups. I find Farmville really annoying and don't play it (or any other Facebook games) but almost all females I spend time around do play it. It's more hit-and-miss with guys.

But is there something inherently immoral or wrong about this pattern? I don't see what it is. It's just a silly game; you might not find it fun to arrange animals and barns on virtual farmland, but that doesn't mean that there's something nefarious or bad about it.