Facebook appears to be testing a new “Want” button plugin similar to its popular Like button.

Developer Tom Waddington from Cut Out + Keep discovered that a Want button has been added to the Facebook Javascript SDK as an XFBML tag – <fb:wants>. The button is not publicly listed among the other social plugins on Facebook’s developer site. Waddington says the button will only work on Open Graph objects marked as “products.”

With Open Graph, developers have been able to create their own “want” actions, but users have to authorize a third-party app in order for those buttons to generate stories on Timeline and News Feed. If the Want button plugin works similarly to the Like button, users will not have to go through the step of authorizing an app. This means even more users will be likely to click it.

Just as the Like button allowed Facebook to collect massive amounts of data about users’ interests, the Want button could be a key way for the social network to collect desire-based data. A Want button plugin will make it easy for e-commerce and other sites to implement this type of Facebook functionality without having to build their own apps. Many of these sites are already using the Like button, but Liking a product could mean users already have it or that they are interested in getting it. Being able to distinguish between these groups of people and target ads to either one could be very powerful for advertisers and help make Facebook a stronger competitor to Google AdWords.

Although publishing “Want” actions is currently disabled by Facebook, Waddington was able to implement a version of the button on his own site. Clicking the Want button returns an error for now.

We’re waiting to hear back from Facebook for more details on when this might be rolled out and how exactly it will work.

[Update 6/29/12 10:33 a.m. PT – Facebook provided the following statement, “We’re always testing new Platform features, however we have nothing new to announce.” ]