1. You famously posited that humans have the cognitive capacity to maintain about 150 stable social relationships. How have tools such as Facebook changed our capacity to handle social connections?

Apparently not at all. It is important to remember that the 150 is just one layer in a series of layers of acquaintanceship within which we sit. Beyond the 150 are at least two further layers (one at 500 and one at 1,500), which correspond to acquaintances (people we have a nodding acquaintance with) and faces we recognize.

All that seems to be happening when people add more than 150 friends on Facebook is that they simply dip into these normal higher layers. If you like, Facebook has muddied the waters by calling them all friends, but really they are not.

This isn’t to say that social-networking services don’t serve a useful function in facilitating our interactions with our “friends,” but what they don’t seem to do is allow us to increase the number of true friends.

2. Does this “Dunbar number” limit how much Facebook can grow and what it can do?

I think its only practical effect is a PR one: you can’t sell Facebook as a way of widening your social circle. This isn’t to say this cannot possibly happen, but rather that it only happens very rarely—and it probably still requires you to get together in person to really create and cement the relationship. Facebook’s functionality seems to lie in its capacity to enable us to maintain friendships through time and over long distances where relationships would normally decay rapidly.

3. How do social-networking tools shape offline social behavior? Could there be negative consequences that we haven’t anticipated?

It’s hard to say, because social networks haven’t been around long enough. But there are two possibilities. One is that time spent on Facebook maintaining old friendships is time that can’t be spent creating new ones. Since friends exist to be shoulders to cry on (metaphorically speaking!) and shoulders that are physically remote aren’t much use for crying on, this might not be ideal. The other is perhaps more serious. The skills that allow us to manage our complex social world are partly learned through experience. If your social experience is largely online (as is becoming increasingly the case with children), then you may not be learning these social skills as well as you need to—that is, if the real learning of things such as negotiation has to be done face-to-face.