The old guy in the corner at a college party can come off as creepy. The same goes for a faculty member on Facebook, the online hangout first populated by students. "Facebook was created as a place for students, not for professors," says Steve Moskowitz, a sophomore at the State University of New York College at Oneonta. Students should be able to express themselves freely there, he says, without worrying what some professor will think. One way to do that is by joining groups. Their names, often clever, mark identities like bumper stickers. Mr. Moskowitz formed the group "Gee, I don't think I want my professors on Facebook anymore." Its icon is a lecturer crossed out with a big red X. He started it when he discovered his music professor on the site. The professor had commented on another group, which students had created to mock his resemblance to a character in a local TV commercial. "This is the funniest group," the professor wrote. "The guy that you say looks like me is my cousin (well 2nd cousin)." The students were mortified. Banter ceased. But like it or not, professors are logging on. The number of Facebook users is doubling every six months, and adults, including professors, are the fastest-growing group among them. Some want to track down students who no longer respond to e-mail. Many are curious to see for themselves the addictive gabfest. As they sign on, they are negotiating the famously fraught teacher-student relationship in new ways. People connect on Facebook by asking to "friend" one another. A typical user lists at least 100 such connections, while newbies are informed, "You don't have any friends yet." A humbling statement. It might make you want to find some. But friending students can be even dicier than befriending them. In the real world, casual professors may ask students to call them by their first names, meet them for lunch, a beer, even. Most still don't think of themselves as pals. Ian Bogost, an assistant professor at the Georgia Institute of Technology, sent "friend" requests to several of his students, but then second-guessed himself. Would they feel obligated to accept? Would they think he expected something from them, maybe more participation in class? It seemed unfair, says Mr. Bogost, who teaches in the School of Literature, Communication, and Culture. "I've definitely kind of backed off the undergrads," he says, "certainly in their earlier years." Nancy Baym worries more about students' expectations of her. A few weeks ago, a young man she did not know tried to friend her, says Ms. Baym, an associate professor of communication studies at the University of Kansas. The same student e-mailed her the next day, asking to get into a class that had a waiting list. He must have thought, "If she's my friend, then she'll let me into the class," she says.