http://www.thisblogrules.com/2010/02/facebook-default-pictures-are-more-fun-now.html

Don't Let Strangers See Your Facebook Profile

Really having problems with strangers viewing your Facebook profile and contacting you all the time. This is how you can keep them all away. Don't let anyone who's not on your friend list see your profile. That stalker and those strangers won't be able to see you or send you messages anymore. From now on, only your friends can see you.

1. On the top of your Facebook page, hover over "Settings".
2. Click on the "Profile" link.
3. The very first setting at the top of the page that says "Profile" should be set to "Only Friends". This will keep everyone except people on your friend list from seeing your profile.
4. Change any other settings to "Only Friends" as you see fit.

http://personalweb.about.com/od/facebookprivacysettings/ht/strangers_off_facebook.htm