Authorities are searching for a tech-savvy mob of teens who used online social networking sites to terrorize a South Philadelphia neighborhood Saturday night, police said. More than 100 youths arranged via unspecified Internet-based means to meet on South Street near Broad about midnight, swarming the area and leading to a melee that ended only after the mob had hijacked a taxi, pulled a woman from her car and assaulted her, and ransacked a convenience store, police said