But the answer seems clear: a member can run a feature designed to find people they know on FB, using their email address book (for a web-based account). They probably don't realize (maybe are not clearly informed) that FB will store the addresses retrieved in a dbase.
So ...
If you are reading this the probability of your internetting becoming part of a file is ever so-slightly increased by talking to US citizens.
Straight forward Set Theory, in other words.