Since 2009, i am using Debian based linux distributions but the most part was on pure Debian and all my linux knowledge comes from Debian.
Debian GNU Linux is one of the first linux distribution and it is one of the most popular. If you take into account the most used linux distributions based on Debian, like Ubuntu and Linux Mint, you can say that Debian is the 3rd most used Operating System in the world!!!
R.I.P. Ian Murdock, wherever you hack.