Many users of facebook do not, in fact, share their face in their profile pictures. They instead opt to share their friends, partners, and hobbies next to their names on their page. They care little about the practicality of their avatars and use the profile picture system as yet another way to share recent events. We may see profile pictures change from cute couples to group shots with friends as breakups occur. Likewise, vacationing users will likely change their picture to include some beautiful landscape they saw in their travels. 

As I look through more and more profile pictures, I find that the "face" is probably one of the least important aspects of the profile picture.