If you've used Facebook messenger for a while, you get used to the Chat Heads feature, and you miss it when you use any other app.
With this free / 99 cent app you can sort of replicate that functionality. Saves you having to remember to keep an eye on your notifications.
You can choose which apps display notifications in the paid version.
https://play.google.com/store/apps/details?id=com.ram.chocolate.nm