You could also look at the FOSS world. Licenses are actually important because if you license some software under GPL, you have legal grounds to sue if a corporation then uses that software in violation of the license. Most corporations just treat GPL as anathema because it's rarely worth the legal effort.

So far we haven't thought of social media content as licensed but it effectively is: the license is usually the EULA of the host giving them the ability to do whatever they want with it.

So really to be responsible with social media content we really should both get a license from our users and then provide a compatible license to federating peers. At that point they can decide not to accept the license, thereby refusing to federate that content. There are already filters like this for filtering certain kinds of content from peers, so it doesn't seem like it should be a huge reach.

Creative Commons might be sufficient here. If you license content as non-commercial, then would Facebook be barred from choosing to display it to the users of their commercial platform?