From now on, everyone in a given region will see the same topics. This is to make sure you "don't miss important topics" that wouldn't otherwise show up in your News Feed, Facebook says. And rather than base trends around activity for a specific article, it's looking at how many articles are discussing the same subject. You should see hot topics surface quicker, and encounter a "broader range" of news than you saw before.
The move should help Facebook expand its Trending section to more countries, since it helps take human editors out of the equation and can shift the attention to local publishers. However, it should also address concerns that Trending can be an echo chamber that reinforces your existing views. Whatever shows up will reflect what's popular for the larger Facebook community, not just within your social circle. This will likely downplay interesting niche stories that previously got attention, but it might just help expand your horizons.