What makes a social network valuable ? Facebook (FB), with more than 1 billion active monthly users posting photos, sending messages, and updating their status, has an impressive market capitalization of $65 billion, or about $65 per user. But Wall Street has assigned a valuation of almost $18.5 billion, or $92.50 per user, to LinkedIn (LNKD), the professional networking site that offers its 200 million members arguably more crucial services, such as help finding jobs. Now a cadre of social platforms aims to disrupt the way consumers share information about personal health, physicians, and treatments. Despite a proliferation of apps that let people monitor every movement and morsel they eat, information technology has yet to revolutionize health care the way it has upended, say, shopping. What the upstarts lack in scale (for now), they more than make up for in utility. Imagine joining an online global community of people with the same rare disorder, or finding a doctor on the basis of detailed patient reviews. Facebook may provide its fans with tools they love, but this new wave of social networks offers tools that its users can’t live without — in some cases literally.
The patient-to-patient network
When brothers Ben and Jamie Heywood, both engineers at MIT, learned that their other brother, Stephen, had ALS (Lou Gehrig’s disease), they were frustrated by the lack of reliable information and support online. In 2004 they launched PatientsLikeMe as a destination for visitors to share personal stories, medical histories, and responses to online questionnaires. Today the site has 200,000 users covering about 1,800 diseases.
Patients aren’t the only ones finding value in the content on PatientsLikeMe. The company makes money selling its users’ data to drugmakers, such as Merck (MRK) and Novartis (NVS), and other research institutions, like universities. Even with all the privacy laws that regulate patient data, PatientsLikeMe, based in Cambridge, Mass., is able to bundle and release its network’s information because, as Ben Heywood says, “we’re radically open about it. We tell our members exactly what we do with their data, where it’s going, and for what purpose.” And the purpose, they argue, is for the greater good: The data can be used to make better, more targeted drugs and more efficient devices. Paul Wicks, a neuropsychologist and research director at PatientsLikeMe, says the company is expanding its patient-driven, standardized questionnaires, and envisions a day when patients can transfer data from health monitors and other devices, such as Google’s (GOOG) augmented-reality Glass product, to create a “learning health care system.”
The doctor-to-patient network
Practice Fusion does not at first seem like a social network. The company provides a cloud-based electronic medical records system for doctors, then sells ads for this platform that subsidize the free service. CEO Ryan Howard knew that doctors would never switch to such a system — even a free one — unless it offered them more convenience. To win over physicians, Practice Fusion threw in a bunch of tools. Most crucially, it allowed MDs to easily transfer medical records to one another. Nearly 150,000 medical professionals are on Practice Fusion, and the service touches nearly 60 million patients. The reason doctors have been quick to adopt the service, which was founded in 2005 and has about $64 million in venture capital funding, is the intra-network information sharing. “That’s the sell,” says Howard.
Practice Fusion has just launched a new service, Patient Fusion, that allows patients to post doctor reviews and check their schedules for an opening before booking an appointment; it’s like TripAdvisor meets OpenTable for health care. New York-based ZocDoc already offers those services free to patients, but doctors must pay the company $300 a month, an amount CEO Cyrus Massoumi says they’re happy to shell out because the service “cuts out paperwork, adds convenience, and can open up their practice to new people.”
MORE: Rethinking health care with PatientsLikeMe
Indeed, the way we find doctors — and our access to them — has always revolved around networks; these new, online platforms simply upend all tradition. As Jamie Heywood puts it, “Social networks have existed in health care for 100 years — as guilds, mailing lists, and simply who you knew.”
One of the newest networks is HealthTap, an online hub of 1.2 million doctors worldwide who field questions from anyone, anywhere. (The homepage provocatively keeps a real-time ticker of “Answers served,” now approaching 670 million.) What works as information delivery to patients is reputation building for doctors, and — just as at PatientsLikeMe and Practice Fusion — the ecosystem offers a trove of data to mine or use to build applications.
The risk with any of the new networks, of course, is that a big social network could decide to leverage its massive scale to enter the health care business. Jamie Heywood estimates that of the 400,000 Americans with multiple sclerosis, 300,000 of them are probably on Facebook, while 30,000 are on PatientsLikeMe. Facebook theoretically could track its users’ behavior to identify those with MS and exploit its position as the largest registry of MS patients in the world. But managing privacy issues and monitoring the quality of the customer experience for hundreds of diseases is incredibly complex. It’s a barrier to entry that health care disrupters are counting on — and what they hope will make their social networks especially valuable.
This story is from the April 29, 2013 issue of Fortune.
An earlier version of this story incorrectly stated that doctors pay ZocDoc a $250 per month fee. The correct amount is $300 per month. Fortune regrets the error.