A friend of mine who is about to become a social worker had a training recently where it was emphasized this wasn't ethical to do, because it messes with the intimacy of the therapy setting because clients should be secure in knowing that it's their choice what to reveal to the therapists.

I only have a skeleton Facebook profile but Linked in pops up my therapist's profile (I think because I email him and my contacts have been imported into Linked in) as someone "I may know" or whatever it is that they say. I would be surprised to learn that my T has seen my profile or otherwise googled me, but I can't say I would be bothered to ask if he had. In some ways, given the many intimate things he knows about me, it's kind of silly to think I would have to shield him from information about me on the internet available to everyone in the world.