As Gizmodo demonstrated with a series of profane and offensive test searches, it will be easy using Graph Search to bring up ill-advised past use of the Facebook "Like" button.
In fact, it's easy to predict that this will become a widespread Internet meme. Jokesters will search for things like "People who like getting drunk and passing out," then post the resulting screenshots of the profile photos and names. The best of these (and by best I mean worst) will circulate virally.
If Facebook is good for bullies, Facebook plus Graph Search will make Facebook great for bullies.
The quality of any search system depends on the quality of available data in the index. And I think Facebook will have trouble with data.
Facebook already broadcasts your Likes to your family and friends, and often gets this wrong. Vegetarians are advertised on Facebook as "Liking" bacon, for example. Even people who have died are showing up as having recently "Liked" something on Facebook.
The other problem Facebook struggles with is fake Likes, where people click Like on things inaccurately, erroneously or fraudulently. The BBC did a test by setting up a fake company and got real likes from fake profiles and fake likes from real profiles.
Graph Search will compound the problem. People who want attention, or to spam or flirt or otherwise get noticed will "Like" everything in sight in the hopes of getting in on the results of as many searches as possible, thereby mucking up the index with junk data.
In general, however, Graph Search makes Facebook a more powerful version of everything Facebook already is.
That’s a good thing or a bad thing depending on what you already think of Facebook.