After a peak in growth rates in early 2009, uptake of Twitter slowed dramatically toward the end of the year, dropping from about a 13-percent growth rate in March to about 3.5 percent in October 2009, HubSpot reports. That in itself represents a huge deceleration from 2008, when growth rates were in four-digit percentages.

 

So what does it mean? Most likely, it means most people who find Twitter valuable already have joined, and that it is a social networking service exhibiting behavior similar to many other social networks. Namely, people sample networks, with a high rate of attrition.

 

Nielsen, for example, has estimated that about 60 percent of new Twitter users fail to return in the next month. That is typical sampling behavior.

 

So the slower growth rates likely mean most people who have been curious about Twitter have sampled it and decided to leave, or sampled and decided it has value. Almost by definition, that means engagement and satisfaction levels should stabilize or grow, since the users who remain are those who find the service engaging and useful.

 

And much of the statistical evidence about user behavior is precisely what one would expect for a long-term user who finds a social network interesting: “friends” or “followers” grow, while the number of posts made also grows.

 

Users are following more feeds, for example, and being followed more in turn. In January 2010, users followed 170 profiles, on average, compared with about 45 in July 2009.

 

Users had an average of 300 followers in January 2010, versus about 70 in July 2009. In January 2010, the average user had made 420 Twitter updates, compared with 120 in July 2009.

 

Twitter users were also including more complete information in their profiles, such as location, biographical information and Web address. Still, 82 percent of Twitter users have fewer than 100 followers, and 81 percent are following fewer than 100 people.

 

That pattern is not surprising. Most, if not all, studies of social network behavior suggest that while many people “read, listen or view,” fewer “write, compose or upload.”

 

At least for the moment, Twitter seems to have reached fairly close to its natural limit of user interest and activity. It is less-popular than email or use of Web browsers.

 

In fact, only about 18 million U.S. Internet users may be Twitter users, meaning Twitter is used by about 11 percent of Internet users. At that level of adoption, Twitter appears to be a useful social network, but not a transforming application for most users.