According to Facebook, the system can:
Add your preferred payment method once then use Facebook Pay where available to make payments and purchases on our apps, instead of having to re-enter your payment information each time
Set up Facebook Pay app-by-app, or choose to set it up for use across apps (where available) -- that means we won't automatically set up Facebook Pay across the apps you are active on, unless you choose to do so
View payment history, manage payment methods and update your settings in one place
Get real-time customer support via live chat in the US (and in more places around the world in the future)
Clearly understand which payment services are part of Facebook
FacebookPay will initially roll out on Messenger and Facebook to US customers this week, and is expected to be initially available for fundraisers, person-to-person payments, event tickets, in-game purchases, and purchases made from selected Pages and business on Facebook Marketplace. Over time, the system will roll out to more users, including usage across Instagram and WhatsApp.
This is seen as Facebook's move to play catch with integrated payment system, but should be taken with a grain of salt as the social media giant is under fire for user data privacy issues.