Facebook (FB) roundup: 1) The WSJ reports about 50 lawsuits have now been filed against Facebook, Nasdaq OMX (NDAQ), and i-banks over a certain May event. Hundreds of arbitration claims could also arrive. 2) Facebook is partnering with cloud storage leader Dropbox to allow users to share Dropbox files with Facebook Groups. 3) Baird is cutting estimates due to slowing PC ad growth, but says checks indicate demand for mobile Sponsored Stories and Offers is picking up. 4) Early results for Facebook's ad exchange appear to be solid.