Facebook appears to be testing a new look for the search bar that more closely resembles the old white search box of old, as opposed to the box-less blue bar of the new Graph Search.
Those who have been switched over to Graph Search since Facebook launched the product back in January have probably recognized that the search box at the top of the page isn't really a search box. It's simply the words "search for people, places, and things" imposed on the blue bar.
This test puts the white search box back into the picture, while still maintaing the Graph Search functionality.
Like any Facebook test (and there are a lot going on at any given time), one can only speculate as to the company's motivations behind it. Is Graph Search adoption falling short of Facebook's expectations? Is it possible that the new Graph Search look isn't prompting people to search - because it doesn't really have a defined search box?
Maybe they've simply received some feedback that this new look is more aesthetically pleasing.
Either way, this appears to be a small test. We've reached out to Facebook for confirmation and will let you know when they get back to us.
And yes, we know that some people still don't have Graph Search yet. These people still see the newer search bar, but without the Graph Search functionality. Graph Search, like the new news feed, and like the new Timeline, is being rolled out very slowly. Facebook says they want to get it right.