I don't use this app, but many of my friends do. To be honest, I never understood it's usefulness. With Apple's new continuity feature allowing SMS messages to be sent and received from your iPads & Macs...are you considering ditching Whatsapp?
If so, think Facebook is concerned?
You misunderstand the usefulness.
First, Whatsapp uses your phone number as your ID. This makes it a replacement for sms in a heartbeat, as opposed to other messaging services.
Second, it's cross-platform, as opposed to iMessage.
Third, it's dead simple to use. Again, as opposed to Hangouts and iMessage. Those are confusing since they deal with phone number and other ID's (Apple ID and Gmail) at the same time.
Fourth, it's got more features than sms. The most important is group chatting. Others are sharing your location, sharing photos and videos, setting group photos, wallpapers, etc.
All of these things are important and Whatsapp has them all. Most other services only have a few of these. Personally, I use Telegram (the app is faster + you can use it on the desktop) which basically is the same thing as Whatsapp.
The thing that should concern Facebook is the social graph. For Facebook itself, it's not a problem since Facebook owns the graph. For Whatsapp, it's different: the user owns the social graph, since Whatsapp uses phonenumber stored in
your phone. Whatsapp does not 'own' users, they simply allow users to find each other. While that is convenient and easy, it also means it can be easily copied. Telegram is a prime example of this: it does the exact same thing as Whatsapp. Furthermore, I don't have to "switch": I can use both apps simultaneously. The cost of chosing another service over Whatsapp is zero.