This by itself is not surprising: as the authors point out, every community, every social network, evinces a similar pattern. We send e-mail regularly to a very small portion of our address book; we call a very small portion of our mobile contacts; we reach out to a very small portion of our Facebook “friends”. This sort of behaviour is true even in other communities; for example, there are a number of opensource projects that behave similarly.But he believes that this number of direct contacts has the potential to be raised in social software. I think he's probably right, but I also think that Twitter's - and Facebook's, LinkedIn's, MySpace's, and others' - status messages serve other purposes.
Here's how I explained it in a recent talk. When I was a kid, I played soccer. And my coach - who was also my father - emphasized that we should communicate constantly. So in our games, we would constantly be calling things out. Often this was encouragement (“good work!”); sometimes it constituted alerts (“man on!”); and sometimes it was just status (“I’m behind you”). But in aggregation, this chatter constituted what we might call ambient status: when the whole team does this, any given player has a pretty good idea of where the other players are, without looking. If I have the ball, and I hear my team's voices, I know where they are without having to scan the field.
For me, the bulk of my Twitter usage is in assessing ambient status. I get a sense of the trends in the fields in which I work, but also the well-being of my contacts. I see when they're engaging in activities similar to mine. I can tell when they're struggling with particular issues. I can get a sense of what they're reading, writing, and studying. I know when they're sick and when they're enthusiastic and when they're uncertain. And sometimes I push out encouragement, alerts, and status myself, not necessarily directed to a specific person, but to the whole ad hoc "team."
So, yes, directly addressed connections are important, but they aren't the point of my individual Twitter use. I suspect this is true for many people who use Twitter heavily - and people who try to use Twitter primarily as a medium for direct connections are consistently disappointed by it.