MEDIAPOST — Facebook and Twitter both saw triple-digit traffic growth on the mobile Web even as MySpace lost ground to its social networking rivals among cell phone users in the last year. Facebook's U.S. audience on the mobile Web more than doubled to 25.1 million in January 2010 compared to a year ago, while Twitter's grew fourfold to 4.7 million, according to data released Wednesday by comScore. By contrast, MySpace's mobile Web audience declined 7% to 11.4 million during the same period...