As part of a technical talk at GDC's Social Games Summit, Amitt Mahajan, lead developer of Farmville, revealed a number of interesting statistics about the game, including current stats of 32 million players per day.
Farmville, a Harvest Moon-like farming social game, is one of Facebook's biggest success stories, earning the small team huge accolades before being purchased by Zynga.
The game took only 5 weeks from conception to launch, developing the back end, using off-the-shelf components when possible, but more than that using best practices from both the web and game development worlds to keep the game extremely portable. Multiple times during development, or even after, Facebook's API or rules changed, and with a strong web development sensibility at the beginning, nothing slowed the team down significantly. This also helped them avoid user fatigue due to lag or errors.
The core team was six web developers, two artists and three designers. After launch, they wound up getting 18,000 users in the first 24 hours. At the end of four days, they had 1 million users per day, all without having ever promoted it.
Now, the game has more than 110 million installs, and they recently breached the 32 million daily user mark, though the "official" number is still 31 million, and as Mahajan says, that number is still growing.