It seems everything between Mountain Lion and iOS 6 will work together nicely. The only problem is Mountain Lion comes out next month, and iOS 6 won't until this fall.
And likely as a result of that gap, Mountain Lion won't see any Facebook integration until then.
Apple billed Facebook integration into its ecosystem as a pretty big part of its keynote presentation at the Worldwide Developers Conference on Monday, but that won't arrive on the iPhone until iOS 6 does this fall. And now a banner on Apple's website says Facebook integration won't come to Mountain Lion until this fall either.