With default settings, anyone can you send friend request on Facebook. This may result in lots of requests from unknowns that may turn your profile look buggy.
To stop receiving new friend requests from unknowns, or to have some control over this, you need to change your Facebook settings.
- Login to your Facebook profile and click Privacy settings.
- Find How You Connect heading, and click edit settings.
- See Who can send you friend request and change settings from Anyone to Friends of friends.
This will stop you from receiving all unknown friend requests. Only friends of your existing friends can send you requests.