Zynga, the social gamesmaker behind Farmville, has a revenue run-rate around $600 million, a source close to the company tells us. Another source confirms that Zynga is doing well over $1 million in revenue a day.

Zynga makes its money getting users addicted to games such as Farmville and Fishville, and then selling them “virtual goods” that make those games easier.

The games are also made easier when a player can get another Facebook friend to sign up and build his or her own farm. This introduces an element of social pressure which makes the games more addictive and more viral.

SUNY Professor A. J. Patrick Liszkiewicz puts it this way:

The secret to Farmville’s popularity is neither gameplay nor aesthetics. Farmville is popular because in entangles users in a web of social obligations. When users log into Facebook, they are reminded that their neighbors have sent them gifts, posted bonuses on their walls, and helped with each others’ farms. In turn, they are obligated to return the courtesies. As the French sociologist Marcel Mauss tells us, gifts are never free: they bind the giver and receiver in a loop of reciprocity. It is rude to refuse a gift, and ruder still to not return the kindness.[11] We play Farmville, then, because we are trying to be good to one another. We play Farmville because we are polite, cultivated people.

As you can tell by the SharesPost chart below, it’s a genius formula that many Facebook gamesmakers are trying, but failing, to replicate.