The "operant conditioning" is spot on however, and as game developers we should be taking notes. The top farmville players spend over $200/mo each on in-game items, that's far more than any multi-hundred-million dollar game could ever hope to make from a single individual. And yes plenty of people don't pay anything, but those free players are valuable assets to the game as they improve the game for paying players by populating it.
I'll try any game out for inspiration, Lord of Ultima by EA was by far the biggest stretch of the term MMO. It's html5 for god sake and uses ajax with a MVC setup. It was soooooo simple, basically just database record vs. database record, absolutely no models or action to code, just statistics vs. other statistics....yet it's successful....it's dumbfounding why people find those types of games fun, but there's tons of money in it.