A couple of months ago, The Economist noted that the Dunbar number appears to apply to online social networks like Facebook.  I’ve since been thinking about the threat this represents to Facebook’s business, and all social networking businesses.

To recap:  The Dunbar number is a theoretical limit to the number of social relationships that one person can maintain – this number is often estimated at 150.  Facebook’s “in-house sociologist” confirmed that the average Facebook user has 120 “Friends” (i.e. other Facebook accounts linked to the user’s account).  Moreover, when measuring the interaction between users, such as comments on each others’ accounts, men average regular interaction with only four people, while women average six people.

You see the problem?  It’s too easy to leave social networks:  you’ll leave as soon as your six closest friends do.  From Tribe to Friendster to MySpace, no one has been able to hold on to their users.  Given that history, Facebook and Twitter have to fight more than just faddishness – they have to fight the cognitive limits of the human brain.

Ironically, social networks do not have the full benefits of network effects.  A really robust network effect means that each additional user of a network adds value to the network for all users.  In social networks, once all of my friends have been added, I don’t really care if any more people join the network.  And that means that the converse is true:  once all of my friends leave, the network has no value to me, no matter how many other users are still on the network.

The ”’Dunbar break”’ occurs at the point at which so many of your contacts have left a social network that you no longer value the network.  Dunbar’s number suggests that this point might be as high as 150, but looking at the actual interaction on Facebook, your personal Dunbar breaking point for Facebook could happen when as few as half a dozen of your friends leave.

That’s why Facebook and other social networks must paddle furiously to try to add value that scales across all users with a true network effect.  But with advertising and applications and ”’lifestreaming”’, they haven’t quite found the magic formula yet.

Does current media darling Twitter hold the key to defeating the Dunbar break?  As a combination of social media and broadcasting, it has some intriguing possibilities.  Ask yourself:  Once all of my friends are on Twitter, do I care if anyone else joins?  And would I care if all my friends leave Twitter, while the rest of the world joins?  A lot of people are answering those questions differently for Facebook and Twitter, which is why Twitter is such a popular dance partner these days.