I am not using FB but just about social media in general, I think for me it would depend on how my therapy with them went and if I liked them. I would not mind and would likely accept the request of an ex-T that was a positive experience for me and/or I am interested in seeing what they are up to in their lives - just like anyone. I wouldn't regard their reaching out any more unprofessional than a former colleague, mentor or student. Current T, I am not sure... it would probably not make a good impression on me that they reach out in any ways that are not professionally-related - in this context, I would see a current T (doctor, lawyer etc) quite differently from a coworker.