I sent this email to Facebook a few weeks ago:
I look forward to hearing from you.
I received this reply:
If you are referring to the "friend finder" feature, we created this
tool to help users quickly find friends who are already on Facebook.
This tool temporarily accesses your addressbook after you have entered your email information and displays the friends in your addressbook who have already joined Facebook. You will also have the option of inviting friends who have not joined Facebook yet. None of the information you enter is saved, and this feature is completely optional; you do not have to use this feature to use our site.
Let us know if you have any other questions.
Thanks for contacting Facebook,
Customer Support Representative