The exploratory effort to share medical-related data was led by an interventional cardiologist called Freddy Abnousi, who describes his role on LinkedIn as "leading top-secret projects." It was under the purview of Regina Dugan, the head of Facebook's "Building 8" experiment projects group, before she left in October 2017.

Facebook's pitch, according to two people who heard it and one who is familiar with the project, was to combine what a health system knows about its patients (such as: person has heart disease, is age 50, takes 2 medications and made 3 trips to the hospital this year) with what Facebook knows (such as: user is age 50, married with 3 kids, English isn't a primary language, actively engages with the community by sending a lot of messages).

The project would then figure out if this combined information could improve patient care, initially with a focus on cardiovascular health. For instance, if Facebook could determine that an elderly patient doesn't have many nearby close friends or much community support, the health system might decide to send over a nurse to check in after a major surgery.

The people declined to be named as they were asked to sign confidentiality agreements.

Facebook provided a quote from Cathleen Gates, the interim CEO of the American College of Cardiology, explaining the possible benefits of the plan:"For the first time in history, people are sharing information about themselves online in ways that may help determine how to improve their health. As part of its mission to transform cardiovascular care and improve heart health, the American College of Cardiology has been engaged in discussions with Facebook around the use of anonymized Facebook data, coupled with anonymized ACC data, to further scientific research on the ways social media can aid in the prevention and treatment of heart disease—the #1 cause of death in the world. This partnership is in the very early phases as we work on both sides to ensure privacy, transparency and scientific rigor. No data has been shared between any parties."

Health systems are notoriously careful about sharing patient health information, in part because of state and federal patient privacy laws that are designed to ensure that people's sensitive medical information doesn't end up in the wrong hands.

To address these privacy laws and concerns, Facebook proposed to obscure personally identifiable information, such as names, in the data being shared by both sides.

However, the company proposed using a common cryptographic technique called hashing to match individuals who were in both data sets. That way, both parties would be able to tell when a specific set of Facebook data matched up with a specific set of patient data.

The issue of patient consent did not come up in the early discussions, one of the people said. Critics have attacked Facebook in the past for doing research on users without their permission. Notably, in 2014, Facebook manipulated hundreds of thousands of people's news feeds to study whether certain types of content made people happier or sadder. Facebook later apologized for the study.

Health policy experts say that this health initiative would be problematic if Facebook did not think through the privacy implications.

"Consumers wouldn't have assumed their data would be used in this way," said Aneesh Chopra, president of a health software company specializing in patient data called CareJourney and the former White House chief technology officer.

"If Facebook moves ahead (with its plans), I would be wary of efforts that repurpose user data without explicit consent."

When asked about the plans, Facebook provided the following statement:

"The medical industry has long understood that there are general health benefits to having a close-knit circle of family and friends. But deeper research into this link is needed to help medical professionals develop specific treatment and intervention plans that take social connection into account."

"With this in mind, last year Facebook began discussions with leading medical institutions, including the American College of Cardiology and the Stanford University School of Medicine, to explore whether scientific research using anonymized Facebook data could help the medical community advance our understanding in this area. This work has not progressed past the planning phase, and we have not received, shared, or analyzed anyone's data."

"Last month we decided that we should pause these discussions so we can focus on other important work, including doing a better job of protecting people's data and being clearer with them about how that data is used in our products and services."

Facebook has taken only tentative steps into the health sector thus far, such as its campaign to promote organ donation through the social network. It also has a growing "Facebook health" team based in New York that is pitching pharmaceutical companies to invest its ample ad budget into Facebook by targeting users who "liked" a health advocacy page, or fits a certain demographic profile.