How is Skype and WhatsApp any different than Facebook Messenger?
It's their Hail Marry as they're getting more irrelevant and losing the 18-46 demographics.
Here's how it's different: Facebook chat isn't just Messenger. It's also going to Facebook.com on your computer and using it there. That's something Skype and WhatsApp don't do.
You can't use WhatsApp unless you have a smartphone. You can't use Skype unless you download, install and run an application on your computer or smartphone. What if you don't have a smartphone (most people don't in my country) and you don't have Skype on your computer? Not everyone is convinced by "oh just download this program, install it, and let it run every time you start your computer 24/7 just so we can sometimes maybe chat". Well chances are you still use Facebook on your computer, because all it takes is to go on a website. No need to install anything. That's why Facebook chat has beaten every other form of chat.