In 2009, the social networking site helped exonerate Mr. Bradford after prosecutors charged him with a robbery in Brooklyn. Mr. Bradford countered that he was at his father’s home in Manhattan at the time. He even had posted a joking complaint on Facebook about breakfast. Subpoenaed records from Facebook backed up Mr. Bradford’s alibi, and the charges against him were dropped. But a year later, while Mr. Bradford, 20, was out on bail on a charge that he had assaulted a relative of his girlfriend, prosecutors hauled him back before a judge to explain a disturbing message that had appeared on the girlfriend’s Facebook account. The posting, written by a friend of the woman, warned her that Mr. Bradford had evil intentions.