US copyright starts at the moment of creation, but the issue here is really enforcement. To get a copyright enforced, you have to bring a civil suit, which is considerably expensive in terms of both time and money. If you're going after a company as big as FB, your expense dramatically increases while your likelihood of getting a favorable judgment drops. And even then, you're probably only looking at getting the content taken down, not a monetary award, because in this scenario it would be near impossible to tell how much ad revenue your specific content generated for FB, or how much was lost to FB if you were getting ad traffic revenue from your content on another platform that's now going to FB.
While these potential problems exist with or without FB in the picture, as any instance owner could theoretically do the same thing, the difference in scale combined with how FB treats its users (cattle) is what's making my alarm bells go off. There's so much potential for abuse, with very little benefit to the existing users of the Fediverse.