Two weeks ago, we noticed that Facebook had begun testing its virtual currency gifting program with branded virtual gifs. In the test, users were able to give friends special virtual gifts – in this case, Mars candy bars like Snickers and Milky Ways – that were also bundled with 10 Facebook credits. Those “credits” are Facebook’s virtual currency, which can then be redeemed in the Facebook Gift Shop.

Today, Facebook expanded its testing of credits bundled with branded virtual gifts to the home page with a campaign for the new movie Terminator Salvation. This is the first time credit gifting has been displayed this prominently that we’re aware of, and if the history of the Facebook Platform is any indication, it’s likely to boost virtual gift send rates significantly. That means more reach for advertisers through Facebook’s virtual gift engagement ad unit – and more revenues for Facebook.

However, Facebook will also have to be careful not to “overincentivize” virtual gifts. Since the Facebook Platform launched, developers have incentivized users to take “viral” actions by rewarding the senders of those viral messages with points or currency in those applications. (This led to a lot of user complaints about application spam, and Facebook has since instituted Platform policies prohibiting app developers from incentivizing the use of Facebook communication channels.) However, Facebook hasn’t gone that far in this case – here, the receivers are the ones getting the bonus currency, not the senders.

Facebook is clearly moving aggressively to test expansions to its virtual currency. Last week, Facebook confirmed to Inside Facebook that it is planning on launching tests integrating its virtual currency into Facebook Platform applications soon. Under such a system, Facebook users would buy currency from Facebook and spend it in applications, whose developers would then get reimbursed by Facebook (less a “transaction fee,” of course) for their work.

In other words, Facebook’s virtual currency is increasingly becoming the basis of transactions inside the Facebook ecosystem.