The purpose of a profile picture on facebook is to attach an image to an account. Should someone search for a person's name, they will see whatever image that person has chosen next to the name. Ideally, that image would be a clear, close up picture of the account holder's face. This would allow other users to identify the account holder more quickly or to differentiate between other users with the same name.

However, not all users do this. Some, if not most, profile pictures on facebook are not simple closeups of the user's face. What, then, are my facebook friends trying to share if not their face?