Good question. When I first started writing, I didn't care much about the platform I was writing on- so I tried Blogger, then Typepad, then Wordpress. Over time though, I realized that having a stable, open source platform that you control is really important. Ideally I'd like to keep writing for the rest of my life- it's hard to know what products and platforms can really survive for a timespan that's that long. Who knows if Medium, Quora, and others will be around?
Because of that, I don't trust Facebook Groups because they might discontinue it in a few years. I'd rather own my own domain, run open source software on my own computing infrastructure. This means I'll have a better shot - certainly not guaranteed - of being around for longer.
The other benefit is that over time, the content on this forum will generate SEO value to my primary domain, whereas Facebook doesn't offer that.