Not only online. If a girl (especially a pretty one) is having a lot of sex, other girls will be the first to call her a slut (mostly behind her back). It has always amazed me how vicious girls are to eachother, even in groups of supposed friends.. guys tend to shoot the shit, but they do genuinely stick up for eachother.

It's a mating strategy. If a girl has acquired a capable mate, then the other females only stand to benefit by successfully undermining that relationship. Upon doing so, the capable mate chances to be distributed across the girl's social network are significantly increased, thus, increasing collective survival.

Then why do we see this behavior in girls, and not boys.

In fact, wouldn't we expect to see this behaviour more in males because males can have multiple mates to a greater extent than females can.

Both sides have their strategies to maximize fertility. This is the female strategy, which enhances the male strategy of sexual aggression.

Citation?

Its actually quite a recognised phenomenon, its just not talked about very often. Not sure why.

http://ideas.time.com/2012/10/04/womens-inhumanity-to-women/ http://www.huffingtonpost.com/dr-robert-lusson/womens-bias-a...

Perhaps because men are generally in power and generally set the topic, and men don't experience being female and doing this.

Alternatively, maybe it's talked about every day but amongst women.

More likely, I think, is that it contradicts the feminist narrative that every gender disparity that disadvantages women is the fault of the patriarchy and every gender disparity that favors women is evidence of inherent female virtue.

That's part of your simplistic narrative, propping up your beliefs. It's not reality.