I got aware of this trend in Facebook just this morning (Dec. 1). I am not sure when my friends started changing their profile picture. I understand the trending by the time I read the status message of my crush.
It says, "Change your facebook profile picture to a cartoon from your childhood and invite your friends to do the same. Until Monday (Dec.6) there should be no human faces on facebook, but an invasion of memories. This is for violence against children."
And I took the pledge. I changed my profile photo with a cartoon character named Stitch.
Then I copied and pasted the same status message on my wall and on my other friends' wall.
I suggest YOU... to change your profile picture as well.