Can someone explain my friends personality?

My friend is sort of weird. She has a really hard time communicating with friends, she purposefully avoids those that know her and acts very fake towards her acquaintances. For example there is this girl she is friends with who she constantly writes to on facebook like "I miss you <3" and stuff, but at the same time she talks to me about this girl like she's a freak, she told me she has really uneven boobs and she walked in on her and her boyfriend doing it anally even tho I asked him and the guy said he would never do that. I'm one of her closer friends and even I get skipped out on, she told me she would talk to me on facebook tonight and she's not even on. I'm not angry because I know that this is a product of her social anxiety. At the same time this girls goes to parties, drinks a lot, and considers herself one of the cooler people. Can someone please explain to me this girl? I know she has issues, she was abandoned by her mother at birth...