You are probably right, as Ubuntu is Debian-based, too.
I do system-administration quite a lot, "make things work" and stuff like that.
I used to use OpenSuSE and when trying to get certain software I found that most software-authors support Ubuntu.
It is really unlikely that someone offers Linux-Software and it does not work on ubuntu, whereas quite often the other way around.
Also, the ubuntu-wikis and forums are really helpful in many ways.
Thus I conclude that Ubuntu might be the most used linux distribution.
Why can't we find out exactly? Because Linux does not "report home" like windows does. I can install 1000 clients with an offline-installer or images, and no one will ever find out if I don't want to.
Personally, I don't like vanilla Ubuntu. I had to help a colleague with some network stuff, he had a new laptop with plain Ubuntu on it. For me, this machine was worse than Windows
(I had the direct comparison, because at the same day another colleague with the same problem and windows came to me, it was much easier).* But if I like it or not, Ubuntu is certainly a big player in the Linux world.
* for those who want to know precisely: I had to find out the WLAN MAC to integrate it into the network. Before fiddling around with menus and stuff, I usually start a console and enter sudo ifconfig and be done with it. On this ubuntu, firstly I did not even find a console. Then I switched to a virtual console just to find that ifconfig was not installed! Having no network at the time of course I could not install it. So I had to go the hard way and find a GUI that showed me the MAC. On Windows, it was Windows-Key - cmd - ipconfig /all ...