Online technol0gy is transformative. It can make the world flat, spark revolutions and even wrap us into personalized filter bubbles. But there’s one thing technology hasn’t been able to do yet: expand our circle of friends.

You’re probably familiar with Dunbar’s “150 rule:” the reason that humans tend to limit the size of their communities to 150 people – whether in prehistoric towns, in military units or in cults – is because the human brain maxes out at 15o friends.

Now it turns out that even Twitter can’t free us of this 150 ceiling. Bruno Concalves and colleagues at Indiana U recently looked at 1.7 million tweeters over 6 months to see how many connections they kept up (connections, as opposed to mere followers, had actual back-and-forth exchanges). True to Dunbar’s prediction, twits generally don’t maintain more than 100-200 friends.

That’s bad news for the filter bubble. You can imagine one hope of avoiding a personalized information bubble is to widen your circle of friends in order to include folks with different viewpoints. That way you might expand the information that arrives on your laptop screen. But, as Eli points out in the intro to his book, merely adding friends to your FaceBook list doesn’t mean you’ll interact with them. No interaction means those “friends” will be virtually invisible on your feed. You’re still stuck in a community of 150.

The only way to truly escape the bubble may be to replace some of your current connections with people who disagree with you – and actually engage them in discussion. Of course, if we’re concerned about the limitations of human behavior, that may be the most pollyanish hope of all.