If you use Facebook and you don't want the whole world to know this, you can limit who can find you in a Facebook search. To configure this, go to Facebook and select the "privacy" link in the upper right corner of the page. On the Privacy Overview page, select the "Search" link. You can then use the "Which Facebook users can find me in search?" dropdown to limit the people who can find you in Facebook. The options are:

Everyone
All my networks and all my friends
Some of my networks and all my friends
Only my friends

If you choose anything other than "Everyone" (which is the default value), you'll get additional checkboxes to further define the users who can find you in Facebook.