If you want to ask about these things, you first should first ask to anthropology how teen girls behave in different cultures before jumping to biological explanations.
We'd want to measure things like "stress" or "anxiety" rather than simply meanness, it's possible this trend is not always expressed through cruelty. (notably, the facebook article suggests explicitly that it's not.)
I think in general the idea of leveraging sociology and anthropology in these cases is perfectly sound. If this only happened in America, I think there'd be a good argument that there is not a genetic component. (or perhaps, that there is a genetic component which is only activated in some contexts.)
One point I'd like to push back on is something I observed when studying anthropology in college. (and to be clear and fair, I am not suggesting that you have pushed this point of view.) Evidence of a single and rare outlier would be brought forward as proof that a set of behaviors did not have an evolutionary basis. Usually this was taken even further: if an outlier could be found, then the entire trait was completely socially constructed. Evolution affects behavior for certain, but these are experienced as drives an impulses which get filtered through the mind, the contextual social, and more. So, if a behavior is very broadly expressed cross-culturally, it is reasonable to suspect there is an evolutionary component to it. (of course this alone does not prove it.) If the outlier scenarios are sufficiently rare, this is likely proof that although there is an evolutionary basis for the behavior, they are not set in stone, and can be suppressed by the right circumstances.