Girls most of the time have a tendency to gossip. We hear something from someone, and we pass it on. Labels are used on people like "slut" and "fake" for the wrong reasons. Most of the time, this takes place in schools, were one girl might tell her "friend" a secret, who decides that it's something really interesting, so she should tell everyone. And she adds in her own twist to it to make it a bit more juicy and entertaining. The next set of girls pass it on with even more of their own versions of what happened, and pretty soon, everyone know about it. The innocent person has now become a "slut" even though she's done nothing wrong. This is how a lot of gossip is spread in school.
But according to the book, girls have been verbally tormenting each other for things they want. Girls have been proven to be the more competitive gender. Before in the olden days, many girls were not free from their parents until they were married. So siblings fought for this freedom by competing with each other for a better husband. Nowadays, we don't fight over these things with our sisters, but with our friends, we fight over many different things. We all want to be better than each other, but to do that, a lot of girls bring each other down using such labels as "slut" and "fake" and many more. If one girl has something, the other girl wants it too, or something better. So we're pretty competitive.
A female brain is more complex than a male brain, and we have been affected by society to behave "sweetly", so we avoid physical fights to appear as that. Instead, we spread rumors and throw around insults to bring each other down and get to the top. This has been a common thin between girls of all kinds, like female apes, girls around the world, and in historic times. This has been a social issue since the dawn of time, and we can't really get rid of it, but we can avoid it, by thinking twice about what we say before we say it.