According to Wikipedia, Just Dance has sold 7,100,000 copies in the US (as of April 2016 - it would've had boost around the time of Joanne and the SuperBowl). Along with this it has 96,000,000 streams on Spotify, and just under 220,000,000 views on Vevo. I'm not sure how many streams a song needs for it to be counted as a sale but surely it has to be close to Diamond? I know all of those streams aren't for the US but I'm sure the US makes up the largest percentage. The last certification it