Apple and Facebook supposedly had a falling out over Ping. It was probably why Ping launched without the rumored ability for users to tie it into Facebook. It was over some API stuff but I think I know the deeper reason why Facebook was not a part of Ping. And it's the same reason why Twitter is a part of iOS 5 but not Facebook. It has nothing to do with personality clashes.
A few months back when Apple announced its subscription plans, publishers were upset. Most people wrongly thought it was over the 30% cut that Apple wanted from each subscription iOS user bought. Rather, it was the private user information lockdown that Apple instituted that publishers had no access to. It was about the publishers' inability to resell or exploit the user information. While it would be scary for a company like Apple to have such a fast database on its users all to itself, Apple has so far not demonstrated that they will give out that information to others the way Google, Facebook, and publishers have.
I think Facebook isn't a part of iOS and never will be until its policies regarding privacy changes. Now, you might way that Twitter can turn around and sell our tweets. Well, our tweets are searching by Bing and Google already. That information is already public. Anyone knows that I tweet about mobile tech, green tech, and sometimes politics. It is not big secret.
In a big way, I'm glad Facebook does not live keep within iPhone and iPad. And I hope Apple will never allow that to ever happen.