> My understanding is it's potentially illegal [1] to pay someone to post and not tag it as being an advertisement.

Yes, this is absolutely the FCC guideline. But this has nothing to do with FB asking for revenue for off-site advertising (which they don't do).

So this part of your comment:

> "Instagram (Facebook) updated their rules in 2016[1] demanding that off platform deals for sponsored content go through them so they get a cut."

is completely wrong.