By Gerry Shih SAN FRANCISCO (Reuters) – Twitter Inc took a significant step Thursday towards broaden its advertising business by offering mobile ads urging people to install apps on its social network as well as through MoPub, the mobile-advertising network it acquired last year. As the mobile app economy grows, app developers have been willing to pay increasing amounts to advertise on major sites like Facebook to boost their app downloads. Twitter said Thursday it could reach 1 billion unique mobile devices through its MoPub network, which places ads inside of hundreds of apps. Twitter, which acquired MoPub last year for roughly $350 million, reaches a more limited audience of 240 million users through its own Twitter.com Web site and mobile apps.