Over the past few months, Facebook has been testing more ways of “seeding” users with Credits, its universal virtual currency, to spend on virtual goods inside social games on Facebook. Just last month, it started testing a Credits giveaway promoting CrowdStar’s Hello City. Now, Facebook is testing more generic Credits giveaways inside social games running on the Facebook Platform.

Over the last couple of days, users have reported seeing the following notice above many Facebook games. Facebook is testing a variety of Credits seeding amounts – we’ve seen from 10 to as many as 25 for some users. (We’ve even seen some users complaining that their friends got more Credits than they did in this giveaway.)

Facebook’s goal with the Credits giveaway is to accelerate the adoption of Credits as a common virtual currency across apps and games. As Facebook CEO Mark Zuckerberg told us a few weeks ago, “It makes sense that there should be one currency. If I go play a CrowdStar game right now and get Credits there, I can’t go use those Credits in a Zynga game, so that kind of sucks. One of the biggest inefficiencies in buying virtual goods is all the friction of having to take your credit card out, so having one store of [virtual currency] that you can use everywhere is both good for users and good for all the apps.”

We expect Facebook to continue doing promotions like these over the rest of the year to build up the number of Credits in circulation. For more background on the Credits rollout process, see our recent in-depth story here.

To dig deeper into the social gaming market, check out our in-depth market research report: Inside Virtual Goods: The Future of Social Gaming 2010.