I searched 'Ubuntu' in Google Trends and this is what I got:
As you can see, the graph is sliding down and its current popularity is close to the levels of 2006.
Back when it was just starting to get popular. I think I started using Ubuntu in 2008.
With the rise of Android and iOS, there has been a shift to mobile OS.
Ubuntu too has come up with a mobile OS. I saw the video demos and it does look interesting.
Surprisingly, Ubuntu is most popular in Cuba.