When Facebook announced its HTC First phone and the Home software, it also released a feature called Chatheads. Chatheads allows you to navigate the Facebook app on your Smartphone without losing track of the conversations you are having with family and friends.
When you start or continue messaging conversation, the profile picture of the other people become round circles that contain the head of the person you are chatting with; thus the name Chathead. You can open many Chatheads and they will stack on top of each other until a new message is received or you click on the heads.
Facebook has now updated the iOS versions of its Facebook app to include the Chatheads.