Facebook and those who don't post profile pictures

I worry about people who never post actual photos of themselves in their profiles. I can only wager that they feel some sort of insecurity, low-self-esteem, some discomfort within themselves and they are afraid to show themselves.
I have fb friends that I've known online for years but I haven't a clue as to what they look like. It bothers me that I can put no face to a name. Seeing is believing, et all.
Are they hiding? On the run from the law? Relatives? Have they done things that they want to keep hidden, secret and unknown.
Do they choose to languish in obsecurity?  Or feel it best not to be seen for who they are? Do they frown upon their physical appearance, believing others will judge them superficially? Are their jobs top secret? Are they involved with the witness protection program?
I wonder