The friend zone does exist, and women will even tell you that it exists.
First of all, most male and female relationships are not platonic, but are grounded. You need to understand this concept first.
Platonic friendships mean that neither party has any sexual interest in each other. But seriously, what's the chance of you going up to an unattractive woman and becoming her friend? This type of relationship only exists with people of the opposite sex that you've grown up with, and view them as family.
But for the most part, our friendships with the opposite sex is grounded. "Grounded" means that the relationship appears platonic simply because both parties know where they stand. One of them may have sexual desire, but won't act on it. One of the 3 things happen to get you in this situation
1) you tried to holla at her, and she said "No" and so you settled with being her friend.
2) you liked her, but never tried to holla at her, and so you and her became friends.
3) you did smash before and even though you want to smash again, but it didn't work out, and so you settled with being her friend, hoping to one day smash again....
It does happen on the reverse, when a man turns down a chick and she stays his friend... but it's rare.
Now that you understand this concept, you can understand "Friendzone"
In order to consider yourself in the friendzone, you must not be content with being her friend. Even if you've stopped pursuing her, if you would smash if she offered it to you, then you are in the "Friendzone". Because you're waiting for an opportunity, instead of creating one for yourself.
The way to avoid the friendzone is to be direct with your intentions, and if she declines, don't be her friend. Move on to the next chick.
Getting out of the friendzone is more complicated. It requires repairing your reputation and her perception of you. This is done, by being confident, meeting and dating other women, and being happy regardless of what she does. Most women want what other women want. If she sees that other women are all on you, she's gonna be like "well that's my friend and I kinda sorta had him first" and then she's gonna put her bid in. But then again, you're still waiting for her to make a move instead of creating your own opportunity.
I just say stay out of the friendzone in the first place.