Earlier quoted context omitted.
But this happens quite often, like every time people get together.
Your saying people who use FB regularly share photos outside of FB and then upload someone else's photos to FB? I'm not saying it doesn't happen, just that it's not the normal path. I think people who use FB generally share photos with their friends who are also on FB, through FB.
The result is basically dozens of photos out of which only the best 2 or 3 get shared on Facebook.
And in general Facebook is not how those photos get shared. Not even its Messenger because it has annoying size limits, which matter for videos. If I were to guess, out of Facebook's properties, WhatsApp is probably the most popular photo sharing app by volume ;-)