Medium is for people that actually read what others write.
On facebook, no-one reads.
This is obvious from the popularity of photo and video posts comparing to the article posts on FB.
So i guess it has to do with what you are searching: if you want good readings, Medium is the right choice.
If you want to chat and share pictures and quotes, facebook is No 1 choice.
I consider moving out of facebook to be the first step for someone to improve his life, but this is my personal opinion and it doesn’t matter to the millions of FB fans.