> Second, it is legal (in the United States) to perform or play music, even copyrighted music, at personal, private, non-commercial events. These events can be in-person or online.
But it is legal for you and Facebook to enter into an agreement to sublicense that content for Facebook's commercial use? You may get the benefit of Fair Use, but the same may not be the case for Facebook, who also must have a valid right to distribute the content.
No. You can't grant a (sub) license of rights you don't have. However, a special licensing regime applies to derivative musical works like covers of songs.
But it is legal for you and Facebook to enter into an agreement to sublicense that content for Facebook's commercial use? You may get the benefit of Fair Use, but the same may not be the case for Facebook, who also must have a valid right to distribute the content.