Facebook has recently been running Google AdSense ads informing viewers that they can “Find Friends on Facebook” by registering for an account. The ads, likely part of a very limited test, could be a sign that Facebook is looking for ways to continue its growth in certain key markets. A strong core user base of influencers is important to maintaining the public perception of Facebook’s as cool and valuable.

The ad, seen by a San Francisco-based Gmail user, reads “Find your friends on Facebook. Join today to connect with them!” The copy implies that the viewer’s friends are already on Facebook, suggesting the ad is designed to persuade hold outs from the service.

This instance may be part of a very limited test. There weren’t readily available mentions of anyone else seeing theses ads. Facebook constantly tests interface changes on its own site, so its reasonable to believe it might do the same in marketing itself.

Some might infer that this is a bad sign for Facebook — that organic growth has slowed and it needs to pay to keep sign ups flowing. However, Facebook hasn’t shown any significant reduction in its growth rate in the U.S. or abroad. It gained 23.8 million new users worldwide to reach 641.1 million monthly actives as of March 1st. This includes 3.3 million more in the US to reach 152.2 million MAU, according to Inside Facebook Gold, our data and research service covering Facebook’s growth, demographics, and monetization in global markets.

Daily engagement is also still holding strong, with Google’s cookie count showing Facebook rising past 300 million daily unique visitors in March.

As penetration rates grow, Facebook may need to try new strategies to lure in those who’ve purposefully abstained from the service. This could take the form of web advertising, more traditional broadcast advertising, or event sponsorship. Its largest marketing effort, though, isn’t costing Facebook anything directly, as over 2.5 million websites now display its name or logo in the form of social plugins.