From Medscape:
But he draws the line at talking about cases with colleagues or sending diagnoses or test results to patients on networking sites. "I can't do any patient care using their messaging or using the site because it's not HIPAA-compliant," Dr. Choi says. "I'll pick up the phone to discuss a case."
Because doctors can be hesitant to share their e-mail addresses -- and regular e-mail is not secure to HIPAA standards -- it's not unheard of for people to find their doctors on Facebook. But the doctors interviewed generally say they avoid making diagnoses or communicating test results over the Internet."
References:
Doctors and Social Media: Benefits and Dangers. Medscape, 2010.
http://www.medscape.com/viewarticle/711717
http://www.medscape.com/viewarticle/711717