> You only see things in your newsfeed from people you've explicitly added to your network (i.e. explicitly opted in to receiving content from).

Eh, I'm pretty sure see random posts from political Facebook groups that were commented on by a Facebook friend I added in like 2006 when that sort of "content proliferation" didn't exist yet (or at least wasn't anywhere near as prominent). I explicitly added that friend, yes, but I definitely didn't explicitly consent to receiving every single piece of Internet content that that friend ever interacts with.