Android users who have seen the new navigation tool are using the beta version of the app from the Google Play Store while those Apple iPhone users who see it have the regular version of Facebook installed on their phone. Android users that already have the Facebook app installed are given a link to download the beta when they install an update for the app. This all indicates that the change is coming from the server-side. And we should point out that the legacy menu button remains on the beta version of the Facebook app.
There is no guarantee that we will see this new navigation bar make the final cut, but for now it is something that is being considered for the iOS/Android Facebook app. If you've been able to test out the new navigation bar, should it stay or should it go?
The beta version of the Facebook app with the new navigation bar (L) and the current layout with the side sliding navigation panel
source: @Numbuh101 via AndroidPolice, MobileSyrup