
As Facebook continues its inexorable growth across the globe, a host of Silicon Valley startups are betting that it's no longer the best place online to hang out with your closest friends.
The latest attempt to gain ground in the social space comes from Treehouse, launching today. It's a mobile app from the founders of video startup Vidly for sharing a stream of photos with an intimate group of friends.
The concept is almost absurdly simple. You take photos, geo-tag and post them and then receive likes and comments from friends. Co-founder Chrys Bader said the idea is to replace MMS, or multimedia messaging, for small groups of friends as smartphones eventually assume dominant market share in the U.S.
Facebook's mobile apps have this photo-sharing functionality too, but Bader says that many users are friends with hundreds of people including work colleagues, long-lost high school classmates and grandmas. (Facebook says its average user has 130 friends.)
"There are all these in-between moments that you might want to share with good friends, but not the world or even your regular social network," he said. He added that Facebook's friend lists offered a far from ideal user experience and that the social network's troubles with privacy issues were eroding user trust.
Several other San Francisco startups are pursuing the same line of thinking. While there are projects like Diaspora that are about decentralizing control over people's social networking information, many entrepreneurs realize that Facebook users just aren't going to quit. While Facebook hasn't announced stats recently, it had been growing at a pace of roughly 50 million new active users every two months into the spring of this year.
Even so, some entrepreneurs are betting that Facebook's dogged ambition to map every relationship of every person will come at the expense of its ability to offer that cozy, sometimes rowdy online experience it once did when people's moms weren't on-board. The more friends people add, the more careful they are likely to become about what to share, even given friend lists and granular privacy controls. People can also be loss averse, so they won't de-friend weak ties even if their presence diminishes the overall user experience.
A few other startups are pursuing a similar strategy. Burbn, a location-based service backed by Andreessen Horowitz and Baseline Ventures, is focused on intimate photo-sharing and life-streaming. Another startup from the original team behind Google Reader, Brizzly, launched a product earlier this month called Picnics (pictured below) for private conversations and sharing.

"There is a need that's not being addressed. I have groups of people that I know very closely like my family or best friends that I want to share with," said co-founder Chris Wetherell in an interview before the company launched the product.
However, preserving intimacy in an online social network poses a number of sticky design problems.
1) How do you know who a person's closest friends actually are? Do you ask them to list 'Top Friends' like on the original MySpace and Facebook apps? Do you algorithmically infer them from e-mail contacts as Google Buzz did or from a person's location history?
2) How do you fight the urge as a business to acquire users as fast as possible and get people to add only the friends that create the most comfortable and intimate environment to share in?
Bader says he's trying to push people to stick to their close friends through the design. The name and look and feel of Treehouse is about being fun and comfortable. He said he's considering limiting the number of friends people can have or forcing them to have two friend groups -- one for close friends and another for everyone. Competitor Burbn has been focused on growing gradually, mostly through word of mouth.
Facebook, of course, has its own solutions. Even if its users have hundreds of friendships, Facebook filters and organizes content partly on how often you interact with the other person through "likes" and comments. It also has friend lists where you can organize groups of contacts like your co-workers or friends from high school.
But "likes" and comments are not exactly a perfect metric for measuring the strength of a bond between two people. And Facebook's new chief technology officer Bret Taylor told us in an interview earlier this month that friend lists were definitely an experience the social network could stand to improve.
It will be interesting to see whether these smaller startups can exploit a new niche or whether Facebook can expend a bit of energy to get its old, unruly feel back. Since all of the experiences these companies offer overlap quite a bit, there's a very, very thin line between success and failure.