Facebook authentication was broken. If you were an early adopter and connected your WordPress.com blog to your Facebook account, you'll probably get some errors at some point (either you've gotten them already or you'll get them sometime soon: maybe both :)).
These errors are because Facebook only granted WordPress.com temporary access to your Facebook account, despite WordPress.com asking for more persistant access.
We've fixed the bug, but if you get any errors, please follow the steps Hanni listed above.
Apologies for the hassle.
If you're still having problems, you've probably hit some other snag, and we can help you if you contact support.
PS: If you authenticated after I wrote this message here, you're all set - the bug was already fixed and you shouldn't get any errors.