(Most) girls are vicious and like to gossip and are constantly comparing themselves to one another. There's even a theory that a hot girl will hang out with all ugly girls just to make sure she is the hottest one of the group when they go out. Of course, this could be pure BS, but my point is that there's a stereotype that girls are catty and always compete with each other. At times, we can all be irrational, crazy, psycho, jealous, insecure, etc. etc.
BUT if you befriend the right girls (the ones without all the drama and vindictiveness), they can also become some of your closest friends. Girls will be able to relate and understand on a different level. You just have to find those girls that are drama-free.
In regards to your friend's girlfriend that you talked to online first...it's much easier for people to be fake online than in person, so maybe she wasn't genuine? Also, is there any reason she could've felt threatened by you?