Social game publisher Zynga said today that its Wild West pioneer-themed game FrontierVille has soared to more than 20 million users since it was launched on Facebook on June 9, 36 days ago.

The game is now the company’s third largest, after FarmVille (62 million users) and Texas HoldEm Poker (28 million users).

As we noted before, FrontierVille is critical for Zynga if it’s going to stay on the treadmill of gaining new users even as its older games lose users.

In the game, the most popular activities include the creation of a virtual family with a customized virtual spouse. Also, the social element of hiring your neighbors to help you work on your homestead has been very popular. Clearly, the game is helping to generate a collective sigh of relief at Zynga’s San Francisco headquarters. Zynga has experienced a decline recently as its core games, including FarmVille, saw drops in users.

Zynga has had a remarkable rise to the top as the largest social app publisher on Facebook with more than 213 million monthly active users. But that number is down from 252 million on April 20, according to AppData. Zynga is trying to reach new audiences by expanding in Japan, publishing on the iPhone, and spreading its games to portals such as Yahoo, Google and MSN. But the core of its effort is to make better games.

Much like the company’s flagship title, FarmVille, the FrontierVille game is a light casual game that you play with your friends. It has a frontier theme where you go out to create a homestead in the wilderness and grow it into a bustling frontier town. Since it’s a family game targeted at everyone, it’s a lot more about tending crops and livestock than it is about shooting guns and scalping your neighbors.

The game is the debut title for Zynga East, the company’s new game studio in Baltimore. Its design was headed by chief game designer Brian Reynolds, a veteran from the traditional game industry who created big titles such as Civilization, Alpha Centauri and Rise of Nations before joining Zynga last year.

Reynolds is as hardcore as they come. He used to delight in setting off nuclear bombs in his game demos. But now Reynolds is going to be essential to Zynga’s mission in raising the bar and attracting new audiences. Zynga has been criticized for copying its way to the top and creating games that — while they are very social — lack deep game play.

“My job is to see what good game design techniques I can bring to bear on social games and to do new, innovative stuff,” Reynolds said in an interview in June.

Zynga said that, so far in FrontierVille, 6.3 million players have built their frontier cabins; 3.3 million have built a general store; 2.3 million people have gotten married; 1.1 million people have had a kid; 10 million people have clobbered a snake (snakes were clobbered 252 million times); and 3.6 million people have scared away a bear.

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