It appears that among the various troubleshooting steps, deleting preferences was not mentioned. Try it. It is usually the first thing to try when FCP is misbehaving.
To delete preferences, hold down command and option while starting FCP X and click Delete Preferences. The application will start in a default state, and open an “Untitled” library.
Reopen the library you were working on, set the preferences the way you like them, and try again.
Also, try exporting a Master File, and under Settings choose H264. Or try exporting to Apple Devices, to check if the problem may be specific to the YouTube and Facebook destination.