Tried to find a working Facebook app for Ub-Mate 16. After a couple of not very good results, I brought up the Facebook login page in my browser and Bookmarked it. So easy and works.
I’m not aware of Facebook having an app in general for the desktop. However, in future, it may be possible to one click install “web apps” from the Software Boutique. 
I don’t even use the facebook app on my phone, not just because of excessive battery drain but their app wants your location all the time amoung other onerous permissions, and I can’t think of anything less desirable than for facebook to have my location.
Not sure I would ever trust a facebook app on my computer.
Of course that’s just my opinion
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Alternatively, if you’re using Chrome, you can click the hamburger menu > More Tools > Add to Desktop and choose “Open in Window.” Store that link somewhere and drag it to your launcher (or leave it on your desktop). It’s kind of like having an app version…kind of…