LOS ANGELES (Reuters) - U.S. television networks face a new challenge in covering this year's excruciatingly close presidential election: prevent closely guarded exit poll results from leaking onto Twitter, Facebook and other social media platforms. The major TV news networks agreed to shield early exit poll data suggesting who is leading in a state until the state's polls close. That means no tweeting exit polls, posting on Facebook, or re-tweeting figures reported by others. ...