Getting any cohort of people to adopt a new social network is hard, let alone doctors, ultra-busy professionals in an industry that is commonly criticized as tech-averse and resistant to change. 

Yet Doximity has managed to sign up more than 400,000 U.S. physicians, about half of those in the country, the company announced Tuesday.

The secure medical network allows doctors to connect with colleagues they know and, in some cases, with doctors they don't know in order to collaborate on a medical case.

The Health Insurance Portability and Accountability Act (HIPAA) prevents doctors from doing this over email due to patient privacy concerns.

"Imagine doing your job without email. You'd be out of the loop within 20 minutes," Doximity CEO Jeff Tangney says. 

The company's DocFax, a HIPAA-secure feature, partly solves this problem by letting physicians share patient files using their mobile devices. That would be helpful, say, if an emergency physician quickly needed to learn a patient's medical history. She could use Doximity to request records from the patient's primary care doctor.

But how many doctors actively use the platform like this, and how many only sign up with Doximity never to return again, is the question these membership reports always raise.

"The 400,000 [figure represents] doctors who have come in and verified their identity with us on the network. It doesn't mean that they're coming back every day. It doesn't mean that they're using us every hour," Tangney says. "But I can tell you we also track that as well. And our engagement, as they call it, is somewhere between LinkedIn and Facebook." 

That translates to somewhere between 30 and 70 percent of users logging in monthly, he says.

Of those two social networks, LinkedIn is the one that Doximity most closely identifies itself with, as the two platforms have similar business models. Doximity charges recruiters $12,000 per year for access to the system. 

For this reason, it doesn't matter whether or not physicians are actively messaging each other on Doximity, according to Matthew Holt, co-chairman of Health 2.0, which produces an annual health care technology conference. 

"Everyone has their résumé on LinkedIn, and they don't use it so much other than to keep their résumé updated. LinkedIn has a very successful business selling access to résumés and access to people," Holt says. "So if [Doximity] has half a million doctors in their books and they have good information about them and they can sell access to recruiters and to employers, that's a great business in and of itself."

Part of Doximity's strategy to sign up new users involves its partnership with U.S. News & World Report. The magazine surveys physicians for its annual "Best Hospitals" guide, and one way physicians can weigh in on the report is by responding to a survey on Doximity. At least one medical center has asked its physicians to sign up for the social network and to take the survey.

Doximity says it's not able to quantify the impact that the partnership has had on registrations. 

Also in collaboration with U.S. News, Doximity launched Residency Navigator, a tool that evaluates residency programs in the U.S. After the tool launched, the company saw the number of fourth-year medical students signed up on the site jump from 12 percent of all fourth years to 40 percent. (These future doctors are not yet included in the current count of 400,000 physicians on the platform.)

Doximity, which is based in San Mateo, California, has raised $81 million to date, and it had its first cash-flow positive month this year. The $54 million in venture funding that it raised in April is still in the bank, the company says. 

Doximity seems to have learned a lesson from the less successful physician social networks that came before it, some of which pursued a business model which provided a marketing channel for drug companies and medical device manufacturers.

Tangney, who founded drug interaction application company Epocrates in 1998 and helped to take it public in 2010, wasn't in a rush to establish Doximity's business model.

"My last company we founded out of my grad school dorm room with the notion that we were going to give a drug reference to doctors and we were going to do it for free and we would figure it out from there," Tangney says. 

"Thankfully, we live in this world in Silicon Valley where you can find people who will pay your bills to try out that kind of thing. And it went very well for us."