Hah, see, the biggest (and probably funniest) love tragedy of my life is that I always fall for girls that give me the friendzone syndrome, but at the same time I've done it myself, many times in the past. Most of my friends have always been girls, and though at first I don't realize it, years afterwards they admit that they 'used to like me' in the past, and that just makes me feel so s*it.
It's like Karma, you know?
But I must say, it never happens 'consciously', or rather, the way it happens with guys. There are no great 'admissions' of attraction from my female buds, after which I reject them with the 'I like you as a friend' line of doom - no, it doesn't happen that way; however, I have noticed the pattern of how it usually works, and it's thus:
If a girl's interested in you, she doesn't tell you straightaway, but it's still pretty obvious (the amount of time she devotes to you, the kind of attention she pays to you, etc.) and if you don't make any advances, depending on the kind of person she is, she might try and 'move on', get someone new, and at the same time keep her eyes on you, trying to see if you're jealous or not; or she genuinely moves on, and you are the one who becomes friendzoned (not completely, of course, if you don't start acting like a jackass, she'll still bear those sympathies towards you). Of course, this is all relative and subject to many varying circumstances, but I've never heard of an instant where it happens the way it happens with men, with the confession and rejection element.
It all comes down to the qualities of the girl, if she's confident, has high self-esteem, she's therefore very proud of herself, very dignified, and she'd usually never put herself out there, because girls are just smarter like that, I suppose. And I say 'usually' since I'm sure that's not always true.
Whatever the case may be with female friendzoning, I bet it's still every bit as painful as the male one.