FarmVille 2 is kicking into high gear as Zynga has begun to open up its viral channels to help the game spread.

Zynga quietly launched FarmVille 2 on Sept. 5. As we noted on that day, FarmVille 2 carries immense importance for San Francisco-based social gaming giant. As Zynga’s earnings took a dip in the second quarter, and its industry-changing “Ville” series of social games began to lose their novelty, the company has a lot at stake with FarmVille 2. At first, the game had a relatively small audience as Zynga’s engineers tuned the game based on feedback from players. Zynga has evidently moved past that stage into turning on its full viral marketing channels, where it cross promotes the game to its existing users.

Now Zynga has 15.2 million monthly active users for FarmVille 2, up 13.5 million from just a week ago, according to market researcher AppData. Just eight days ago, FarmVille 2 had only 1.7 million monthly active users. On a daily active user basis, FarmVille 2 has 6.9 million users. FarmVille 2 is now the fastest-growing game on Facebook.

The original FarmVille grew its audience to as 83 million monthly active players. It introduced social games to the mainstream and became one of the most broad-reaching titles in history, even as critics said it wasn’t really a real game. So the remake is akin to Zynga’s best shot at launching social gaming 2.0, and game designers like Zynga’s Tim LeTourneau said that created FarmVille 2 to become a lot more addictive and game-like.

Zynga now has more than 344 million monthly active users on both Facebook and mobile platforms. FarmVille 2 is Zynga’s ninth-largest game at the moment. The top of the heap is Zynga Slingo, with 53 million users. ChefVille has 50.2 million users, and Texas HoldEm Poker has 39.3 million users. I’ve been playing the game and have gradually climbed to level eight, and I have a real farm ecosystem going where I can generate a lot of income by growing my ingredients and the using them to create high-value foods for the village market.

The new FarmVille 2 has better graphics based on the Flash 11 format (and Stage 3D) that Adobe has created to improve animations in web-based games. The characters and animals are created using 3D graphics though they are displayed in the familiar isometric, two-dimensional perspective. The animations look more fluid, and the landscape is more interactive as you move your mouse cursor over the screen. FarmVille 2 is an attempt to capitalize on how web-gaming technology has moved forward in just three years.