http://aswathdamodaran.blogspot.com/2013/10/twitter-announces-ipo-valuation.html
Here's a link to Prof. Damodaran's website, which as all sorts of useful valuation information:
http://pages.stern.nyu.edu/~adamodar/
Personal note: Prof. Damodaran received his MBA and Ph.D. in Finance from UCLA. I was an undergraduate student at UCLA at the time that Prof. Damodaran was pursuing his graduate degrees at UCLA. Prof. Damodaran was the teaching assistant for one of my Economics courses, and I still have his handouts from that class.