I had never heard of it, but it had over 100M views on YouTube." How could a song be so popular, and yet unknown to me?" Its YouTube views represented its popularity. To give you an example: in researching this article, I stumbled across a song, "Princess of China", by Coldplay featuring Rihanna. You can see the number of streams on Spotify (and, doubtlessly, it's calculated into the Nielsen "streaming" rating, along with YouTube and Apple Music, among others), but YouTube is still the one that represents a song's popularity to the national consciousness. It would be ludicrous to even suggest that someone go out and buy an album (except for Adele's 25, a weird aberration that somehow got people back into fyi or Barnes and Noble or wherever music is sold).
I believe YouTube streams are the "box office earnings" of the future. Wiz Khalifa’s Furious 7 anthem has had a surprisingly long shelf life, in part because the video pays tribute to the late Paul Walker. Imagine how many times people who REALLY like Taylor Swift must've watched it.
I personally must have seen the "Blank Space video" at least 25 times, and I don't even consider myself a die-hard Taylor Swift fan. If you factor in all the grown ups who don't care, the people too young to be interested, the people without access to internet (let alone wi-fi) and the people who have never even heard of some of these artists, you aren't really left with all that many people. Videos having 1 billion views doesn't necessarily mean 1 billion discrete people have watched the video.