If you look around the blogs there's lots of speculation about complexity of integration, history of sour negotiations over things like Ping.
I don't think it's anything like that - here's my really simple list of reasons:
Facebook is essentially an "Application" - it collects content, stores it, aggregates it, distributes it. It allows users to have "space" on the web - a virtual home, so to speak, albeit connected with their circle of interests.
This is nothing like what Apple/iOS needs. As a device and operating system, iPhone/iOS needs connectivity; it needs conduits for information - channels if you like. That's what email is, that's what SMS is, that's what instant messaging is.
That's also what twitter is - twitter is not an application as in the sense above; it is a transport hub, a conduit for realtime data to flow between relevant users. It might be described generically as "social media", but its modus operandi and purpose are entirely different to facebook. Twitter is a way for information, of almost any type, to flow between users. Facebook on the other hand is designed as a place to land to consume content.
That's not to say that Twitter doesn't intend to move up the "value chain" to become a place of higher value consumption - indeed, its acquisitions of the likes of tweetdeck and the long overdue enhancements to its online experience, such as the tie in with photobucket, clearly signal this intent.
But quite simply, the DNA of twitter is more akin to the phone line as Facebook's is to the phone.
There is a complete cultural mismatch between Facebook and Apple. Apple lead the way on user experience and strive for total customer satisfaction. Apple's mantra is to put users first. Apple is slick and consistently good. (Yes, they've had their hiccups, but they deal with them sensibly.)
Little could seem to be further from the truth for facebook. Facebook acts first in its own interests, then retracts if the backlash is sufficient. Their mantra is clearly "It's easier to ask forgiveness than permission". Facebook is clumsy and self-centred.
Shoddy user experience and scant regard for user privacy have been demonstrated by Facebook time and time again. These two brands do not make good bedfellows.
And that's it - these two reasons alone, in my opinion, have killed for the forseeable future any likelihood of Apple and Facebook joining forces. Personally I see it as no great loss.