It lasted for several months, but Facebook finally updated its iOS applications to support higher resolutions of the larger iPhone XS Max and iPhone XR phones that Apple released last fall, allowing Facebook to work on the device's native resolution rather than just expand iPhone XS size one through 9to5Mac.
The update also adds support for large screen sizes for the latest models with the 11-inch and 12.9-inch iPad Pro, which were released last fall, removing the black bars that the application used to display. (Although the usable application space is essentially the same.)
Updated iPhone apps get a bit more space due to the update. 2688 x 1242 iPhone XS Max (3x pixel on the effective screen) and 1792 x 828 The iPhone XR screen (2x pixels) effectively results in a screen display equivalent to 896 x 414, slightly higher than the 812 x 375 dimensions XS and X screen.
In related news, Apple revoked the Facebook development certificate yesterday, in response to the social network abuse of distribution licenses for adult and teenager tracking, disabling Facebook's ability to develop new versions of its iOS applications, preventing the company internally testing newer hardware.