Okay, I say "linux". I admit it. However, if I'm writing something up, like documentation, I'll use its full "You're in trouble mister" name, "GNU/Linux". But when I'm just talking, it's far to much of a mouthful, and far too vestigial to worry about much. GNU/Linux is its full and rightful name, but I would also compare it to always referring to the car you drive as "2001 Ford Focus ZX3".
Look at it this way, if you will: If you know enough about it to know its full name is GNU/Linux, then you know it is anyway, so you shouldn't need to fully qualify to say it.
And the corollary: If you don't know it is full name is GNU/Linux then you probably don't know what the GNU Foundation is, and so having people add "GNU/" to the front of it will just serve to confuse and irritate you.
I've often wanted to ask a question of RMS: "Sir, if you insist that everyone call it 'GNU/Linux', do you also insist that everyone address you 'Mister Richard (whatever the M stands for) Stallman' without exception? By your own logic, you should."