It's taken the US Patent and Trademark Office four and half years to consider it, but Facebook now finally has a patent on one of its central features: photo tagging. Applied for in October 2006 and just granted this week, this legal doc gives Mark Zuckerberg and a couple of his buddies credit for designing a method for identifying users in "a selection of an item of digital media." That could be photos, video, audio, or text -- the main drive of the patent is that it lets people associate a given chunk of media with a person and inform others of this association. The wording of Facebook's claims is rather specific -- you have to, for example, allow the identified person the opportunity to reject the identification -- so having this patent need not necessarily preclude other sites like Flickr from engaging in similar, but not identical, behavior.