This has happened to me too. Until a few days ago everything was fine, now Facebook and about 5 twitter accounts do not receive posts.
I’ve tried deactivating the whole plugin, reinstalling an older version, and just deactivating/reactivating Publicize. Disconnecting reconnecting with WordPress.com and NOTHING WORKS.
The only thing I can’t do is delete my accounts to re-register them because neither Facebook or the Twitter accounts can be deleted. It just keeps saying that I’ve connected so I can’t delete them to see if setting them up again works.