Honestly, I'm beginning to think that maybe up/downvotes shouldn't federate, with each instance having it's own score for each post. Though maybe that makes the score-based sorts less useful on small or private instances. Or alternatively, each instance could send a single up or down vote based on the ratio of voted from it's users if any had interacted with something, to let vote score be mostly coming from within one's instance but still with some amount from outside to sort the timeline with. Could make it take more effort to manipulate the rankings since one would have to either make bots on many different instances or set up multiple whole instances for it instead of just bots on a single one.