I probably wouldn't do it as that would put me in a big group of people who are T's FB friends--family, personal friends, etc.--and that is not who I am. It would be uncomfortable seeing his personal posts about his goings on on my wall. Would not want that! But I might send a connection request on a more professional, career-oriented site, such as Linked In. People don't post personal stuff about themselves there, so you can maintain a connection with your therapist without it being too intrusive. Does your T have a presence on Linked In?