Having done many fb apps and making heavy use of twitter bootstrap by myself for other projects recently (oh god, app development on lightspeed!) i first looked at it thrilled: hey cool- this will come in handy! But after about 5 seconds: wtf?
This is just twitterbootstrap painted blue. Using this for your facebook apps would NOT give them a genuine facebook look but make them only look cheap.
The typo is not right. Font sizes, line spacing - everything. Forget about the columns. They are twitterstyle, not facebook. Buttons? They are grey and blue, yes - but not facebook style. Tables? Not at all. I could continue this list for every single element! Facebook has unique tabs, unique breadcumbs. Completely missing!
My quick advice for getting a nearly facebook-like style in your appe in seconds: embed their CSS in your header. When you investigate a bit how they build their page elements, you can mimic their style really quick.
And for the "fb bootstrap": nice idea. Awful implementation. Stay away from it. But everyone has to decide for himself - seems like most people like it, tough.