When I met with Doximity CEO Jeff Tangney a few weeks ago in San Francisco he said his company was about to cross an important milestone. He wasn’t kidding.

More than half of all U.S. physicians are now members of Doximity, a social and professional network for doctors.

Tangney said doctors have a distinct need for a secure platform where they can communicate with their peers. They talk about a lot of things, from employment to health news to patient care issues.

They use the network for decidedly clinical things, too — out of necessity. At many clinics and hospitals the sanctioned modes of communication among doctors and other care givers are woefully out of date and inefficient. At some hospitals, doctors still use pagers. So many are taking the problem into their own hands, Tangney says, and using Doximity as a secure and private “back channel” to help them communicate and coordinate care.

After doubling in 2013, Doximity says its growth accelerated in 2014 to more than 400,000 members now, making Doximity the largest secure network of doctors. The network was launched in 2010.

San Mateo, California-based Doximity says physicians have made 21.2 million professional connections on Doximity, averaging 53 colleagues per member.

“As it stands now, physicians are forced to waste countless hours on redundant paperwork, faxes, phone tag, and pagers. With Doximity’s secure email, scheduling, CME, groups, and much more in the works, we’re doing our best to help,” Tangney said in a statement today. “Our dream is a day when physicians have better communications software than teenagers, but we still have a long ways to go.”

Doximity has built a number of specific services around its social network that are useful to doctors. These tools include an online professional CV, a secure digital fax service, a personalized news feed. Members can receive targeted career opportunities based on their location, schedule, and compensation level.

The service also now offers a public ranking of medical training programs, which can be used by med school graduates to choose a residency program.

Doximity raised $54 million in venture funding in April 2014, bringing the company’s funding total to $81.8 million. The company says it hasn’t touched that money since it had its first cash-flow positive month earlier this year.