I would find it very unlikely that an ethical therapist would friend random people (or anyone they do not know) on Facebook. It's very likely that the person sending the request could be a client (or friend of a client, or a client posing as someone else)-- and most T's would avoid that. 400 friends is not a particularly high number for a working professional who probably has colleagues, friends, family, and former school aquaintances from each degree & city they lived in. If you're dead set on messaging your former T on facebook, then send a message--- but I would not send a friend request. It's just poor form for a former client. The vast majority of Ts would find that to be a conflict of interest, and would deny the request and probably feel uncomfortable about it. I know my T has mentioned getting those kinds of requests from former clients and greatly dislikes it and always denies them.