It’s not surprising, then, that Facebook has turned into a gathering place for strangers sharing their deepest secrets. Emotional-support groups have sprung up around topics broad and narrow: diabetes, addiction, egg donation, a specific birth-control device now pulled from the U.S. market, parenting children who might grow up to be psychopaths, rare diseases that affect only a few dozen patients in the whole world. The internet has always promised to connect people by common interest rather than geography, and with its 2-billion-user base, Facebook is where those connections are often being made. “For people searching for support, [Facebook] is a one-stop shop,” says Andrea Downing, a moderator for BRCA Sisterhood, a support group for women who have tested positive for breast-cancer mutations.

Downing carries a mutation for BRCA1, which can raise the risk of breast cancer to more than 70 percent. Finding that out was devastating. “I did not know anybody who was going through the same experience,” she says. “When you can’t even talk to your own friends and family about what you’re going through, just living with that is really hard.” She eventually found out about BRCA Sisterhood on Facebook, where she suddenly found a few hundred women who understood exactly what she was going through. The women, she says, were a “lifeline.” They divulged their anxieties. They shared the latest research. They posted photos of their preventive mastectomies. BRCA Sisterhood has now grown to 10,000 members.

NGLY1 Families, on the other hand, has fewer than 60 members on Facebook, and it doesn’t expect to get much bigger. That’s because NGLY1 deficiency is extremely rare, known to affect only a few dozen patients. With those few dozen scattered around the world, Facebook is a vital node for connection. “This is pretty standard now. You would be hard-pressed to find a rare disease without a group,” says Matt Might, whose 10-year-old son has NGLY1 deficiency. Might is a member of multiple Facebook groups for NGLY1 and related diseases, where members support one another through health crises and share hard-won medical information about the rare disease. Recently, a member posted about a medical emergency, and Might told me he was able to direct the person to the right doctors.

Since Facebook has pivoted to groups, it has added several tools for group admins, including ways to filter membership requests and delete content from banned members. Most important, perhaps, it made the membership of closed groups private. Until earlier this year, nonmembers could see who had joined a group even if they could not see the posts inside. (Secret groups are unsearchable, and their membership lists have always been private.)

This had created obvious problems for support groups, which want to be findable but don’t want to broadcast their members’ private lives. Last year, Catherine St Clair decided to start a support group for people whose DNA tests revealed unexpected biological parents, after meeting another woman in the same situation on Facebook. St Clair created a closed group because she wanted other people to find it. And, of course, she invited the other woman. This was before Facebook made the change, and her membership quickly became public. “When she realized that, she dropped out real fast,” says St Clair.