Physician Advisors aka the early adopter 'brain trust'...

Jeff Benabio, MD, @DermDoc

Dermatology 
Kaiser Permanente San Diego
Blogger, The Dermatology Blog  

Shelley Binkley, MD, @HealtheWoman

Obstetrics & Gynecology
HealtheWoman

Aaron Blackledge, MD, @CarePractice

Family Medicine
Founder/CEO, Care Practice Medical Group

Kent Bottles, MD, @KentBottles

Pathology 
Health Care Futurist/Consultant

Jen Dyer, MD, MPH, @EndoGoddess

Pediatric Endocrinology 
Nationwide Children's Hospital & The Ohio State University College of Medicine

Natalie Hodge, MD, @NatalieHodge

Pediatrics
Chief Health Officer, Personal Medicine 

Howard J. Luks, MD, @hjluks (Originator 'do we need a physician TweetChat')

Orthopedic Surgery
Chief Sports Medicine, University Orthopedics, PC & CMIO, iMedExchange 

L. Gordon Moore, MD, @LGordonMooreMD

Family Medicine
Ideal Medical Practices 

Mike Sevilla, MD, @DoctorAnonymous 

Family Medicine
Social media enthusiast 

Bryan Vartabedian, MD, @Doctor_V

Pediatric Gastroenterology 
Texas Children's Hospital, Blogger 33Charts