I am not FB friends with my T either. I feel like it would put her at risk and complicate my therapy. If I started to post depressing things and she didn't notice, perhaps she could be held accountable if I ever had a serious breakdown. I don't know for sure but it's not worth the risk. Also, people have known me and my humor for years often misinterpret some of my posts and if she did that and brought them up in session I feel like it would only complicate things.
In theory, I might be willing to become FB friends with her after I've finished therapy but I'm not sure.