Well, if your email address is in the address books of those extraneous people you know, and they have at some point used Facebook's "friend finder," a feature which peruses their address books for matches with name/email address combos already in the FB database, then when the would-be friender entered your name/email address combo, FB likely matched it up with info already in the database ("hey, we already have an artie505@whatever.net—in the address books of Paul Extraneous and Art Extraneous—so let's try to get these good folks together on Facebook").
One can choose to find this unauthorized bandying about of one's email address sinister (although the friends/acquaintances who used "friend finder" provided authorization by proxy; does that make them sinister?), but with 10% of the world's population using Facebook, techniques for figuring out who really knows whom might, alternatively, be regarded as essential.
Anyone who's tried to find his or her long-lost college roomie Bill Clark, only to give up after the first four or five hundred Bill Clarks returned by a Facebook search, might appreciate seeing the correct Bill Clark proffered as a potential friend, without being too alarmed by the implications such magic carries regarding the data manipulation going on behind the scenes.
