Most fellow devs I've spoken to that work in mobile apps say they're really impressed by Microsoft's tools, and they're the best tool for cross platform work. Problem is though that development has to start with Windows and port to other platforms for the best results. Most start with iOS and Android and then maybe their business reluctantly thinks about Windows at a later date but wants it at the lowest cost and the devs don't have the budget and time to learn the MS tools.
On the subject of Facebook, oddly their own app for Win 10 desktop is actually pretty good and even includes live video feature. I guess though it's still not UWP, else they could have easily knocked out a mobile app that is basically the same but formatted for mobile. Instead they've given us a god awful port that's unusable due to instability and functionally lacking.
Microsoft's own app on the other hand, while I used to moan about it for missing features, is infinitely superior to Facebook's effort. Why can't Facebook just pay MS to update the app? It's in both their interests and especially Microsoft's if they want to launch a Surface phone or whatever. It will tank if the core popular apps are garbage conversions that crash all the time.
Instagram app is a little better, although still bloated and slow to launch. It is also very unstable on an SD card. 9 times out of 10 I can't get it to post an image without crashing, so give up. Have moved it to phone memory now and see how that goes, but on a 640 phone memory is very limited.
Anyway, MS's Facebook app I can no longer install. I was able to install but it would only run so long as I remove the Facebook version, but now I can't even install it from the store. No install button at all. Annoying as I removed it so I could clear the memory and try to install it on phone memory, as the 'move' option said not enough memory (despite it claiming it was taking 40mb and phone memory has 1.5gb free supposedly). So if you have the MS app, don't uninstall it! Or if you do, use the FB web site. It seems to be more responsive than any of the apps anyway in Edge. Just you won't get live video, contact syncing etc.