My two bestfriends are fighting, and i'm in the middle?

So i have have two best friends, lets say their names are Blair and Serena (GG names are all i could think of) so, We are this laid back 15 year olds who do nothing but do our little slumber party at home. We haven't been friends for a very long time and we had our pasts. I was this girl who always got drunk a year ago and so was Blair but Serena, she haven't tried those things yet. We always tell her dont do it cause its just gonna give you problems with boys and etc. But she did anyway, she got kinda tipsy with some dudes Blair used to hang out with and shes doing the middle finger in front of the camera. I talked to her the next morning and told her to never do those things again especially with people she doesn't know. And Serena promised she wont. But Blair on the other hand got really mad. She didn't speak with Serena for a week and Serena didn't know why. I just told Serena maybe Blair's just stressed. And the next day, Blair attacked Serena on FB. She told her that she's acting like a ***** doing those things in the camera. That she's changing a lot. That she's so proud about drinking at age 15. That drinking with those people is so stupid. (since blair had history with one of the guys. she loved him dearly) and now Serena is so upset. They dont want me to be involved but they secretly want me to take sides. i can feel it. Serena doesnt know what to do. She thinks it doesnt really matter and Blair's just making a fuss about it. She told me she just wanted to experience it but Blair's just screaming at her in Facebook. Blair told Serena she acted like **** and Blair defended herself saying it was just one time. What should i do?