Leila is a sex worker. She goes to great lengths to keep separate identities for ordinary life and for sex work, to avoid stigma, arrest, professional blowback, or clients who might be stalkers (or worse).
Her "real identity"—the public one, who lives in California, uses an academic email address, and posts about politics—joined Facebook in 2011. Her sex-work identity is not on the social network at all; for it, she uses a different email address, a different phone number, and a different name. Yet earlier this year, looking at Facebook's "People You May Know" recommendations, Leila (a name I'm using using in place of either of the names she uses) was shocked to see some of her regular sex-work clients.
Despite the fact that she'd only given Facebook information from her vanilla identity, the company had somehow discerned her real-world connection to these people—and, even more horrifyingly, her account was potentially being presented to them as a friend suggestion too, outing her regular identity to them.