Facebook’s Credits virtual currency continues to expand. It brought on a huge new payment service provider, PayPal, last week, and it began showing apps that use Credits within the “Featured” column of its new Games dashboard. We’ve also been hearing rumblings from developers about expansion, with the possibility that Facebook might somehow make Credits compulsory.

This weekend, Ryan Spoon of Polaris Venture Partners spotted another way that Facebook appears to be giving Credits new emphasize: running in-house ads on users’ home pages. Per his screenshot, the headline of the ad says “Happy Island & Credits,” and the text highlights the fact that you can “[c]reate the island of your dreams with Facebook Credits!”

We asked Facebook about the ads, and here’s the response:

As part of our continued testing with Facebook Credits, we’ve started a small amount of targeted user education to help people easily identify where they can interact with Facebook Credits, and better acquaint them with these applications.

Happy Island was launched by CrowdStar last December as the first game from a big developer to exclusively use Credits. The company has since made Credits the sole virtual currency for its other games, including Happy Aquarium and Happy Pets. And, last week, Facebook appeared to only be showing CrowdStar within the “Featured” column — today, however, we’re seeing other games appear here, that have Credits as one of several options.

It’s not clear how much home page advertising Facebook plans to do for Credits, nor if these sponsored ads will also show games that only have Credits as an option. These on-site promotions all appear to be in the early testing stages — which is how Facebook describes the point Credits is at, overall.