By your email address; most likely when you signed up you let Facebook go through your email/address book to search for people/friends that are also on FB.

They also look for connections, multiple friends that have the same friends. So they conclude that there is a very likely change you know that person as well.

They got it down to a science...

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).