By Jennifer Saba and Alexei Oreskovic NEW YORK/SAN FRANCISCO (Reuters) – Photo-sharing service Instagram unveiled a new feature on Thursday to let people send images and messages privately, as the Facebook-owned company seeks to bolster its appeal among younger consumers who are increasingly using mobile messaging applications. Facebook Inc, the worlds No. 1 Internet social network bought Instagram for $1 billion in 2012. The new features comes as Facebook and Instagram face increasing competition from a new crop of mobile messaging services, including WhatsApp and Snapchat. In October, Facebook said it was seeing a decline in daily use by younger teenagers in the United States, although it said overall use by teenagers was stable.