There's a rather amusing guide to facebook portraits here: http://www.fastcompany.com/1692957/facebook-profile-picture-flowchart
Amusing because it's not only funny, it's also somewhat true. Spot on, in fact. What it doesn't cover, however, is the use of avatars - pictures that aren't portraits.
That's OK - I can cover that in a few lines:
#1) He/She is wanted by the police or doesn't want to be recognized and identified, which would result in them being wanted by the police.
#2) I'm butt-ugly and would prefer a picture of a potted plant to my actual face. So would my friends.
Now recently on facebook there was a little "viral" campaign that saw a lot of users replace their photos with cartoon characters, ostensively to end child abuse.
I'm not making this up.
Purportedly you were drawing attention to the plight of children all around the world, and marked yourself as someone not only politically correct but as morally superior to all your friends who didn't have a cartoon avatar (obviously pedophiles, every one of them, probably a good time to drop them...).
It gets better.
Fox News (or some equivalent) picks up the story and suggests that the cartoon avatars is just what the pedophiles want, as now they can message their little friends with complete anominity.
Remember, I'm not making this up.
I have something to say about this. What can I say about this? How do otherwise perfectly intelligent people get on board with something like this? Where, exactly, is that leap of reasoning that goes "If I just replace my facebook photo with a picture of a cartoon character that I like it will help to end child abuse..."?
I mean, really?
Probably you have a few friends that did this. Sadly, me too.
Me, I'd just stop communicating with them for a few months, then drop them on the Facebook friend swap.