Houston surgeon Rafael Lugo needed help. He had a patient with a tumor and didn’t know what to make of it. He showed it to oncologists and surgeons in his hospital, but they were stumped as well. Lugo could take a biopsy, but wanted to avoid that given the risk factors.

So he turned to crowdsourcing, and posted about the tumor on Doximity, a social network exclusively for physicians. Soon someone chimed in, and referring him to an MD Anderson surgeon who had seen a similar tumor. It was benign, and Lugo was advised to just monitor it.

“We were very grateful. We were looking at major operation but that would’ve been overkill,” Lugo said.

That was all made possible by Doximity, which announced Tuesday that it’s signed up more than 400,000 U.S. physicians, about half of the country’s active physicians.

“Who really does need to communicate quickly, urgently, securely about how to solve a problem? It’s physicians,” said Doximity chief executive Jeff Tangney. “Right now they’re very siloed off.”

The social network for physicians has reached its size in just over three years of existence. Tangney credits the sites’ commitment to make physicians more productive. It offers encrypted communications to meet privacy regulations. Physicians who attempt to join the network have their identities verified before being given access.

“The Internet is a tool that so far has been built for shouting for people to put it out there publicly on Twitter or Facebook or somewhere else. We’re teaching the Internet to whisper,” Tangney said. “Doctors do need to whisper about patients. They do need to be able to share and collaborate about a patient without everyone being able to see it.”

Doximity’s success is also a reminder of the power and value of social networks. While Facebook has run away with the title of being the premiere general-interest social network, smaller more specialized networks have sprung up and found success. LinkedIn caters to job seekers and networkers, Github unites computer programmers.

Doximity raised $54 million in venture capital funding in April of this year, so will have the capital to continue to grow and provide helpful digital services for physicians.