The icon for Messenger consisted of a blue dialogue bubble with a gray bolt.
In 2013, the logo was changed to be a much brighter blue and the bubble is ellipse-shaped rather than a rounded rectangle.
The icon was modified making the dialogue bubble, and the bolt within, much more rounded than before.
The icon was modified to have a gradient from blue to red via purple. This is to reflect the merging of Instagram messages and Facebook Messenger.