The 21k reviews is for "this version" -- there's actually nearly 2 million reviews for "all versions" with an average of 4 stars.
The Android app has 3.2 million reviews (averaging 3.6 stars - 600,000 or so are 1 star).
So it doesn't seem as bad when you consider "all versions" instead of just "this version". I wonder if it's a matter of more tech-savvy people reviewing the newer versions when they come out? Or it could just be that the app really is getting much, much worse with every release...
The app used to be all native objective-c. It was actually pretty good. They made a switch to make their different platforms share more code and be more consistent with each other and quality took a HUGE nosedive.
They stated they would never do that though. Because that will allow you to export emails from Facebook to Gmail. Facebook said they are protecting you by not letting you export them to Google. Never mind the fact you can do that to Yahoo with the click of a button.
The sync worked before because Google allowed them to use a special API to allow them to show contacts on your phone without actually syncing them to your account (removing the Facebook app removed the contacts)