One merged list let the busiest network drown out the rest. Now each network gets its own section, so a quiet one is still visible.
"Viral in your niche" used to be a single merged list, which meant whichever network we happened to have most data for filled it and the others were effectively invisible.
Each network has its own section now, ranked by that network's own engagement rules, because likes on Instagram and saves on Pinterest are not the same unit and pretending otherwise produces a ranking that means nothing.
When a network has nothing matching your niche tags in the last month, it falls back to your niche across all time, and then to that network's overall top posts, with a badge saying which. You get something useful instead of an empty box, and you can tell which one you are looking at.