The writer of www.zdnet.com Steven J. Vaughan-Nichols has done research and found 5 most popular Linux distributions:
2: Mageia: Unlike Ubuntu’s Unity user interface, Mageia uses the Linux pro friendly KDE 4.8 desktop;
3: Ubuntu: it has relatively new Unity interface;
4: Fedora: Red Hat’s community Linux distribution, uses GNOME 3.x;
5: Debian: which just turned 19, remains important because year after year it continues to be loved by its users.