The title and summary confused me and made me want to read the article, breaking the Slashdot tradition. Is the discovery that black holes are merging generally, per how the summary starts off "now we have evidence that hundreds of thousands of them are merging". Or is it that black hole pairs are merging, and the summary says shortly thereafter "the population of massive black hole pairs that are merging"?
Or is it that the way that the general merging is happening is one pair at a time, such as with Sagittarius and Andromeda, which just happen to the the two that are merging the closest to us, and which will result in a binary system forming? will the binary system then merge with another supermassive black hole? Or does it have to then merge with another binary system? Or is is a one level merge event, and once every supermassive black hole has participated in one merge event, then it's all done?
I got confused enough and got so any terms thrown at me in the summary hinting that something got grossly oversimplified in the title.