> But the content that they’re trying to protect isn’t theirs -- it belongs to their users.

Kinda. Yes, Facebook says that content belongs to users (otherwise they'd have harder time explaining they are not liable when it's illegal), but users also agree to give Facebook “non-exclusive, transferable, sub-licensable, royalty-free, worldwide license to use any IP content that you post on or in connection with Facebook.”

For example, if a user deleted their* content, Facebook can still use it and show to their friends. That's why it's "kinda".