Interesting piece from the New York Times on the emergence of social networks like Path and Pair which aim to better reflect real world friend groups and sharing.

FACEBOOK makes sharing easy — too easy, some would say. Because one’s social network often consists not only of actual friends but also relatives and sort-of friends, along with sort-of friends of their sort-of friends, you need to be careful about what you post. Yes, the site does allow you to define smaller circles of friends, but that requires constantly monitoring what should — and should not — be shared with whom.

All of this has created an opportunity for start-ups to offer sharing that is intimate by design.

Newer social networks, like Path,FamilyLeaf and Pair, offer a range of constraints. A Path network, available only on smartphones, has a maximum of 150 friends. FamilyLeaf is restricted to family members. And Pair, which like Path is for smartphones only, is as small as a social network can be: just one other person.

The average Facebook user in United States has 245 friends, according to a study published in February by theInternet and American Life Project of the Pew Research Center. That figure, however, well exceeds the “Dunbar number,” the 150 meaningful relationships that Robin Dunbar, an anthropologist, has suggested is the effective neurological limit the human brain can handle.

Dave Morin, who worked at Facebook for four years before leaving to help found Path in 2010, explains the rationale for his company this way: “Facebook has made socializing on the Internet normal. But now there is an opportunity to return to intimate socializing.”

Mr. Morin says he called Mr. Dunbar, a professor at the University of Oxford , to find out more about his research and theory. Mr. Dunbar told him that social networks resemble a set of concentric circles: 150 people constitute the outer boundary of friends, 50 is the limit for trusted friends, 15 for good friends, and 5 for best friends.

When Path introduced its social network, it capped the number of friends at 50. Today, Path has more than a million active users, and the average user has 40 friends. Last year, Path raised the maximum to 150 because, Mr. Morin says, the users “like the headroom.”

Still, the core proposition is unchanged. No subgroupings are permitted, and there are no privacy settings to adjust. Close friends are in; everyone else is out. “You’ll never experience the problem of accidentally sharing something with the wrong people,” Mr. Morin says.

FamilyLeaf limits an individual’s network in a different way: it is just for relatives. This start-up, founded just eight weeks ago by Wesley Zhao and Ajay Mehta, both 19, received financing and guidance from Y Combinator, a seed fund in Mountain View, Calif. After the site opened, demand soon exceeded capacity, and there is now a waiting list to join.

Mr. Mehta says he and Mr. Zhao have used Facebook solely with their friends. When their older relatives want to join in, to stay in touch, a problem can arise: a moment’s inattention when posting can lead to accidental sharing with those one doesn’t intend to include. “I don’t want to have to filter myself for two completely different audiences,” Mr. Mehta says.

Mr. Zhao adds: “It’s no more feasible than thinking that you could use Facebook for both friends and work. It’s as if there would be no need for LinkedIn.”

Each family’s network has one designated gatekeeper, though one person can belong to more than one family’s network.

FamilyLeaf has streamlined photo sharing, which can be done simply by sending photographs in an e-mail attachment to the family photo album maintained at the FamilyLeaf site.

“My mom is technologically unsavvy,” Mr. Mehta says. “She can’t use Facebook, but she can use FamilyLeaf. Same with my grandparents in Mumbai.”

Pair offers a way to share with one, and only one, person. It was first financed by Y Combinator, and Pair’s five founders spent the first three months of this year in the San Francisco Bay Area, working with Y Combinator’s partners. The start-up’s founders are from Canada, and three had girlfriends who remained there. They created the Pair app to communicate with distant loved ones without having to worry about misdirected messages.

Jamie Murai, one of Pair’s founders, offers an example: In text-messaging, “you have to scroll through a list,” he says. “Sometimes you think you’re sending a message to your girlfriend, but it’s actually going to your workout buddy. You can be confident that once you tap on the Pair icon, everything you share is only shared with your partner.”

Pair users can send a text message, photograph or video, but the application also has some distinctive additions to the usual menu. The pair of users can create a drawing together simultaneously, share a to-do list or press a button to let one person know the other’s location. Pair also offers “thumbkissing”: Pressing one’s thumb on the screen, aligned with the image of the other’s, causes both phones to vibrate.

Pair’s mobile-only app was released last month and had 100,000 users after only seven days, according to the company. (At present, it is available only on the iPhone.)

If a relationship changes, the app can be paired with a new most-important person. “Initially, we focused on the romantic relationships,” Mr. Murai says, “but, really, it’s turning out to be for the one most important relationship in your life, which could be your best friend or your son or daughter.”

These micro- and supermicro-size social networks aren’t competing directly with Facebook or even with one another. Conceivably, one could be active on all of them. But then we may bump up against a new neurological limit: the maximum number of social networks that the human brain can handle.