Facebook is an accelerant. The movie "Mean Girls" largely predated social media and highlighted many of the same issues. This seems to be a pretty ingrained issue, although I think most people agree that Facebook makes it worse.
Here's a theory that's nearly guaranteed to land me in hot water:
Why are young girls so awful to each other? I don't think it's unreasonable to suspect that is has to do with "hormones," new emotions, and new social awareness, etc. But, it's also the case that from a strictly evolutionary perspective, young girls are are the most fertile and therefore the most desirable. This will decline over time, and sharply as women enter their 30s, 40s, and 50s. Perhaps there's an evolutionary reason why young girls work so hard to identity and sideline any competition? If this is the period of their life where they are the most desirable, there might be an evolutionary benefit to be the most cut-throat when it comes to vying for the best mating opportunities.
A few caveats:
- Hopefully I don't have to explain that evolutionary impulses are not the same as desirable or positive social values.
- Nor are evolutionary impulses immutable, or always expressed in the same way. (for example, sports are often observed as a peaceful replacement for warring city states.)
- Further, I'm definitely aware that evolutionary psychology can sound very reasonable while being perfectly rubbish and unscientific. There are plenty of people who say things like "because hunter-gathers experienced X," that explains "dubious trend which I have anecdotally observed." Even in the case where an evolutionary psychology explanation may happen to be correct, it remains difficult to prove or test.