See: Reverse Email Search with Facebook
A simple Google search for "inurl:www.facebook.com/p/" suggests that there are over 26.5k public profiles on Facebook already.
There's more - outsiders or non Facebook members can use the public search box on Facebook's home page to find friends who may have public profiles on Facebook and contact them.
How Do Search Engines Discover Facebook Profiles ?
Normally search engines like Google or Yahoo! discover new web pages only when they are linked from another website that's already in the index of search engines.
Would anyone have a clue how Google would discover your Facebook public profile when it's not hyperlinked from any other site.
Related: Most Innovative Facebook Groups