I got "married" with Facebook for the first time in August 2016. After a short-lived "married" life with it I had to get "divorced" from it to prevent something precious from deterriorating. In retrospect I can realize now how mindlessly I used Facebook, controlled and subjugated by my ego, and writing and reacting on the autopilot mode, as it were. I was less aware that I was even less mindless back then when I was offline, especially when I was under the influence of alcohol, which happened more and more frequently. All these accumulated mindless behaviors of mine were powerful enough even without Facebook to destroy that precious thing I tried to save by quitting Facebook slowly but steadily.
When I had to decide to get "remarried" with Facebook this July for some professional reason, I told myself to try my best to use it as mindfully as possible. Since Facebook doesn't allow us to use only business pages, I decided to start using it again for personal purposes, though rather hesitantly and cautiously, restricting the so-called "friends" to those I know personally offline.
Except for the originally planned professional use, whose effectiveness I'm not so sure of yet, the two main private uses of Facebook I benefit from most are 1) following posts by some spiritual teachers such as Eckhart Tolle as well as some rabbis and Jewish organizations, and 2) participating in two "groups", or online communities, one of which is by Eckhart Tolle himself, to socialize with like-minded people from all over the world, though I participate passively so far.
As Eckhart Tolle points out correctly in What Do You Think about Facebook, Facebook is also an excellent place for observing various online manifestations of the egos of its users, including, of course, mine. The most frequent and "popular" type of posts seem to be those meant for mutually receiving instant gratifications from "friends". This seems nothing but - sorry for the expression - online m*st*rb*t**n. Instead of reacting to them no less mindlessly, I simply observe them and mindless reactions they generate as precious opportunities to practice mindfulness.
This time I also stay away from initiating or being dragged into political or religious arguments, in which nobody can convince nobody else with a different opinion. This is even more vane than posts for instant gratifications. In this "remarriage" of mine with Facebook I seem to be far more successful so far than in my first "marriage" in tackling this double task of using Facebook both mindfully and as a field for practicing mindfulness.
PS: The above is a hopefully conscious self-observation. ;-)