By Alexei Oreskovic and Gerry Shih SAN FRANCISCO (Reuters) – More than 100 million people will be glued to their TV screens on Sunday, when the Denver Broncos take on the Seattle Seahawks in Americas premier sporting contest, the Super Bowl. But two fierce rivals, Facebook Inc and Twitter Inc, will also be clashing head-to-head on a second screen that TV viewers will tune in for the big game. The two social networks are jostling to be the venue of choice for fans to comment on big plays, the star-studded halftime show and of course the commercials – multimillion dollar productions by major brands that are often a draw in their own right. The Super Bowl is the biggest stage for a broader battle that has intensified over the past year between the two behemoths of social media.