You're supposed to someone when their story is bullshit. In other words, you're using that wrong.

You use "cool story bro" when someone is talking bullshit, if something is tl;dr, or if someone tells you something/complains at you and you don't care.

For example: You would say "cool story bro" to try and save face for being wrong and getting corrected, and it'd still be out of context.

That was a really cool story you just told, bro.

no, you use it when nobody cares.

just ta help this guy out, i wanna say that i care.

hey guy you dont need to be a jerk... what the hell am i doing half of funnyjunk are jerks..

I hate my life...

I hate your life too, dude.

Oh, god that reaction pic is awesome.

No, I'm using it right, according to your definition.

Why do you think my story is bullshit...

oh dear...