Farmers of FarmVille who are looking to purchase in-game currency will now have a new option. Apart from the previously  available options, Credit cards and PayPal, Farm Cash and Farm Coins can now be purchased using Facebook Credits – Facebook’s virtual currency.

This new payment system have been adopted in a number of popular Facebook games like Happy Island, Happy Aquarium and Happy Pets, where Facebook Credits is the sole option for in-game purchases.

This new payment option in FarmVille is arguably bigger for Facebook Credits than it is for the game, with the amount of exposure it will get through the game.

FarmVille has more than 83 million users, making it larger than Twitter. If the Facebook Credit payment option catches up in FarmVille, Facebook could very well cross the speculated $1 billion revenue mark in 2010, taking into account the 30% take on revenues generated through Facebook Credits.

Paypal on Thursday said that it had teamed up with Facebook to use PayPal as a means of currency for ads, as well as for Facebook Credits, Facebook’s virtual currency.

For more than a decade, Paypal’s focus has been on payments – making it easier for customers to send and receive money online in 24 currencies and 190 markets around the world. We’ve always been an important part of the developer ecosystem on Facebook and with today’s news, we’re really excited to expand our relationship directly with Facebook to help grow their advertiser and developer businesses,” writes Osama on the PayPal blog.

Advertisers would be able to pay for Facebook ads using PayPal money transfers, which the company said would help small international advertisers. Previously Facebook advertisers needed to link a credit card to their accounts in order to place ads on the site. The options will be launched within the next few weeks.

PayPal will also become a payment option for Facebook’s virtual currency, Facebook Credits, which is currently being tested in a small number of games and applications. Users will now be able to buy virtual goods using their PayPal account.

“We’ve added another form of payment: PayPal. Facebook and PayPal are establishing a long-term, strategic relationship to simplify payments globally for advertisers purchasing Facebook Ads. PayPal will also be available for users buying virtual items in games and applications. By teaming up with the global leader in online payments, we are able to provide users, developers and advertisers a fast and trusted purchasing experience across Facebook,” writes Facebook in Facebook Ad’s Notes today.

With this partnership that brings together the world’s largest social network and the world’s biggest online payments system, PayPal may very well become the de facto payments system of Facebook and the online world.