Except each instance has its own URL meaning ranking for ANYTHING is extremely hard since each domain’s rank will always be weak in the sea of others. Each is even being penalised by the algorithm if there are duplicate content mirrored between different URLs. It’s the weakness of the fediverse if we are to follow how search engines have worked in the last decades. Maybe it will lead to new search engines (I hope so) but right now it is not going to work well to replace for example Reddit … or rank well in general at all.