Whatsapp and Facebook Messenger are the most important app on our phone. It is now more easy to use this apps now. You can use these 2 messengers inside one app,it's called "Disa", and you don't need to switch the app while the messages come. You can see and chat your facebook and whatsapp friends inside Disa app. The application is coming soon in iPhone too.Android App
The application is available in playstore so there is no need to doubt the security of the app.
See Also : How to use Disa app in android