"Facebook, wisely, never relinquished control over their platform."
What did Facebook do differently? Third-party Facebook clients have been possible for quite some time, perhaps from the initial launching of their API.
I've tried various Facebook iPad apps and they all sucked. I think it's simply that Facebook has so many features and changes at such a rapid rate that it becomes unfeasible to write an app for it. Compare that to the wealth of Twitter- and RSS clients and weather apps. They're all gorgeous front ends around conceptually simple data.