The Like button has been available since late April, but Facebook and some of its advertisers are taking it a logical step further in a test rolling out today: Like buttons in banner ads.

So if you’re reading an article a news site that runs banner ads for large advertisers like Mountain Dew and JC Penny,  you might start seeing the Like button appear within those ads. Click on it and, just as with the usual plugin, your Like will be recorded back on Facebook in your friends news feed and on your wall. As usual, your friends who have Liked the ad will appear on the widget, along with a count of the total Likes; you’ll also appear once you click, for others to see, and the item you Liked will appear in your Interests on your Facebook profile.

Interactive ads have been around for years, and Facebook itself has long provided social context advertising for brands and other advertisers on its home site. What’s more interesting about this integration is the fact that there’s Facebook’s social context appearing alongside clearly branded content off-site. Third parties have tried similar types of social advertising content for the web, such as SocialMedia.com’s “People Powered Ads” introduced last year, that have typically relied more heavily on Twitter content.

A Facebook spokesperson tells us that the companies are giving Facebook some of the inventory on the ad for the social plugin iframe. Facebook then serves the social plugin into ad, the advertiser’s server then serves the ad, pings Facebook, and Facebook provides the relevant social context information.

While many people have expected Facebook to provide its own advertising network for the web in addition to the advertising options on its site, this is one of the first instances of the company doing so — albeit in a very limited, incremental way.

And in terms of revenue, the advertisers are “not paying for the test but there will be a revenue model when we launch more broadly,” according to the spokesperson.

One final note: Extending social plugins into advertising has also been done by publishers, such as The New York Times, without Facebook’s involvement. In the second screenshot, from an article, The Times has placed its own ad in the popup window for Facebook’s Recommend button, and includes text that says “FACEBOOK TOOL SPONSORED BY.” The in-ad social context, of course, is not there.

Learn more about how advertisers and content publishers can implement Facebook’s social plugins in the Facebook Marketing Bible.