Social game maker Zynga is rolling out the first major expansion for its uber-popular FarmVille game, which debuted back in June 2009 and has been played by hundreds of millions of Facebook users.

We reported last month that Farmville was preparing the expansion, called FarmVille English Countryside, and it finally started rolling out to fans tonight. (At least I received my first invitations from friends tonight). Zynga typically rolls out its new games slowly to make sure everything is working right before it begins heavily promoting the game.

This expansion is the biggest single change that Zynga has made to FarmVille. If Zynga gets this right, it could return FarmVille to its glory as the game with the biggest and most profitable audience in the world.

The Facebook game is not exactly a sequel, as it uses the same Facebook application as the original FarmVille. It will allow fans to create a parallel farm alongside their existing one. But the new farm is entirely immersed in pretty English country landscapes and themes.

After I accepted my invite, I landed in a farm that was in need of repair. As soon as I restored some crops, I was asked to send passports to my friends so they could join as well. Then I was asked to customize my farmer (and was given an opportunity to purchase a variety of clothes with Facebook Credits). Then I returned to fixing up my farm. The more I send messages to friends, the faster I can fix the farm and move on to the English Countryside expansion. Zynga certainly knows how to make its games spread like lightning.

A central character to the expansion, “The Duke,” has given me some missions, such as writing post cards to friends. (The game crashed once while I was sending cards).

I’m looking forward to meeting some English sheep. If there’s a big deal in this game expansion, it’s the sheep. Yes, while the original FarmVille lets you breed horses, in this game, you will be able to breed sheep. You can combine sheep of different colors and you’ll never guess what the resulting color will be for the baby lambs. Todd Arnold, general manager of FarmVille, told me in an earlier interview that this is an “innovative feature” of the game. I call it a mini-game.

For most hardcore gamers, such puffery in the form of breeding sheep is what they hate about FarmVille, which drew considerable criticism as a farming simulation because it didn’t have much game play. But casual players loved it, and the game hit a peak of 83 million players in March, 2010. FarmVille is one of the most successful games in history, surpassed only by Zynga’s CityVille, which got as large as 100 million players. Mark Pincus, chief executive of Zynga, owes his billionaire status to the success of FarmVille.

FarmVille became the talk of the industry because it proved that traditional game companies that focused on hardcore gamers weren’t thinking big enough; it took a scrappy startup like Zynga to imagine and create FarmVille and market the hell out of it to a receptive audience of older women. (Although maybe “imagine” isn’t the right word, since the inspiration for the game came from earlier games such as Farm Town, Chinese farm games that came before that, and the original farm game, Harvest Moon on the Nintendo DS).

FarmVille has been in a steady decline as Zynga focused on launching other games such as FrontierVille and CityVille. The game now has 47 million monthly active users and 13 million daily active users. The decline was so steep that writer Peter Yared insisted that Zynga had better launch FarmVille 2.

In FarmVille English Countryside, Zynga has added more story and more game play to address not the critics, Arnold said, but feedback from fans who wanted more things to do than raise crops. At the beginning of the new expansion, you meet the Duke, who flies in on an airship with a British flag on it. You have to repair the Duke’s airship and then his farm. Then you have to invite some Facebook friends to a bon voyage party (yes, that’s Zynga’s not-so-subtle way to make the game viral). After that, you can fly in his airship to England.

There, you can start farming your own plot with all-new crops, equipment, buildings and decorations based on an English theme. But Arnold said that the good thing about FarmVille English Countryside is that it does not force fans of the current game to start over. All of your FarmVille coins and cash, your gift box, farm size and your level of achievement can transfer over to the English Countryside game. (I can see that my coins and my achievement level are updated).

The missions and quests are similar to the story-based play that you can pursue in FrontierVille and CityVille. In this case, the Duke periodically shows up to give you a task to do.

Zynga is promoting FarmVille English Countryside with its own fan page. The FarmVille fan page has more than 30 million “likes” on Facebook. But the English Countryside fan page already has 2.1 million “likes” with little promotion.

Arnold thinks the new game will be popular because many players needed a second farm to express themselves. The company tested a number of concepts for a theme, and the English Countryside theme rated the highest.

Zynga won’t say how many of its 1,700 employees worked on the game or how long it took, as it considers that a corporate secret. The FarmVille studio in San Francisco worked on it; Manuel Bronstein Bendayan was senior director of product on the title. The team is blazing a new trail with FarmVille and had many discussions about when to launch an expansion. Clearly, though, as users defect to CityVille, FarmVille needed its makeover.

“The initial idea was to give people a fresh start with their farm,” Arnold said. “We wanted to take it further than that, adding characters, story and more guided game play.”

But the team wanted to preserve the free form, open-ended game play that made FarmVille so popular from the beginning. Just about every week, the team adds new features to FarmVille to keep users interested, and it will do the same for the English Countryside.

And of course there was the sheep-breeding innovation. A farm hand character named Angus shows you how to do it. You can pick a ram and a ewe and then hope for the best while you wait to see what color combination results. You can speed the process along by offering the sheep a “love potion,” which is an opportunity for Zynga to sell you a virtual good. So far, there is no advertising built into the English Countryside expansion.

Another feature that Zynga added was a progress map that shows where you are in the different levels of the game. Now you can see exactly which friends are at the same level as you are and where you have to level up next. You can also help your friends get to the next level.