Users on the Social media report that they found a special WhatsApp button in the "main" Facebook app for Android. When you click a button that acts as a shortcut, the WhatsApp window opens directly on top of the Facebook application.
If you are in the "control group" of Facebook, the button should appear at the top, right under your name. For the testing period, the function is available to a limited number of users and only for the Android version of the application, but can later become available for iOS users.
According to one of the users, the WhatsApp button appeared after he switched the language of the Facebook application to Danish. Not surprisingly, that the company already picked Denmark to test different developments - for instance, its "Bonfire" application for the video sharing.
Recall that this is not the first time that Facebook is testing with the integration of products of its "subsidiaries" in the main Facebook application. The company executed a similar function for Instagram earlier this month.