Attorney here! (Not legal advice, seek counsel if you need legal advice.)

You’re confusing ownership with distribution and possession. Ownership is about the rights you have to exclude others from your work (i.e., copyright). When you transfer a copy of your work to a third party such as Facebook, you don’t grant them ownership or even an ownership interest. You’re just granting them to right—-but not the obligation—-to republish and retransmit your work to others.

Since it is a right and not an obligation, Facebook also has the right not to store or republish your work if they desire. But you still own your content and can otherwise do whatever you like with it.

Whether you keep a copy after you give it to a third party is your choice and responsibility.