By changing a privacy setting, you can limit the likelihood of receiving Facebook friend requests from total strangers.
Do you get friend requests from strangers or people you hardly or don't want to know? Unless you embrace all comers on Facebook, these requests are nothing but an annoyance. One likely source of such requests is people seeing your profile in the People You May Know box that sometimes appears to the right of the News Feed.
By default, Facebook broadcasts your profile to everyone, but you can restrict your appearances in the People You May Know box to only friends of friends. Here's how:
Go to your Facebook privacy settings by clicking on the downward arrow in the upper-right corner and selecting Privacy Settings. Scroll down and click the Edit Settings link next to How You Connect. For the question, Who can send you friend requests? select Friends of Friends and then click Done.
Now, only friends of your Facebook friends will be able to send you friend requests, and according to Facebook, your profile will appear in the People You May Know section only for people who are friends of friends.
Via Here's The Thing