Facebook messenger for iOS was designed mainly to accommodate devices with smaller displays and since then iPads had to install the same app designed for iPhones and iPod touch’s which basically enlarge the whole messenger to fill the iPad’s large displays.
However today, Facebook release the official native messenger app for the iPad which utilizes every available pixel of the iPad display. splitting the screen into 2 panes which shows the list of threads on the left and your current conversation on the right.
The Messenger for iPad includes all the features found in Messenger for iPhone and iPod Touch including stickers and VoIP calls. Newer additions has not yet been supported such as the split screen selfie camera and the tap-and-hold video recordings but is expected to make it through the next updates.
Facebook have not mentioned any plans for their Android variant at this point but given the iPad treatment, it won’t be long the Android version will have the same update as well.