Apple has discontinued the Facebook and, I believe, the You Tube direct interactions. The procedure now is to share to your desktop and from there upload to the destination website.
When you share to your desktop initially an unplayable temporary file will appear until the export fully completes, at which time the temporary file will disappear and be replaced by a fully complete movie file. Sometimes the process may take a minute or so, depending on the size of the exported file. Is this possibly what you are observing? If so, the full file should be displayed somewhere on your desktop.
If the above is not your situation, try deleting preferences, as that procedure sometimes cures export issues. To delete preferences, open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if you can export a full file. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
-- Rich