@Gargron sorry to be a dick about it, but it's Mastodon that proliferated the behaviour of sending activities to all the instances that your users follow other people on. So it's entirely possible that at least one from the 10M users will be followed from every instance in the fediverse, therefore when someone posts, Meta's servers will federate with *everyone*.

@liaizon @chrismessina @tonyg @alexeheath