Weeks after attending a pro-Palestinian demonstration, Egyptian-American student Layla Sayed received a text message from a friend drawing her attention to a website dedicated to exposing people it says promote hatred of Jews and Israel. "I think they found you from the protest," the friend wrote. When Sayed visited the site, called Canary Mission, she found a photo from the Oct. 16 rally at the University of Pennsylvania with red arrows pointing to her among the demonstrators. The post included her name, the two cities she lives in, details about her studies and links to her social media accounts. Canary...