The other key is the Twitter ecosystem keeps evolving, too. Twitter the Service hasn't changes substantially in the last two years, but Twitter the Culture has. People who gave up on it a long time back do find their way back to try it again, discovering that more people are using it in their social networks with better tools than they had when they left. The same will be true for all the people who just came on board to be best friends with Oprah.

It may be more accurate to look at those numbers and suggest that Twitter's adoption cycle is often quite a bit longer than with other SN. Despite all the help the 140-character constraint and simplicity of the service brings, members have to get over thinking of Twitter as just a broadcast of the mundane. Having an account already created makes it that much easier to join in earnest later.

I've noticed many of my old high school friends (from the 1980s) are going to Facebook when they start diving into social networks. Twitter is still a big conceptual leap.

You also get a plug at the end of chapter two of their "other" Twitter book, as a resource for good information about Twitter.

I think it would be wise to update this article to emphasize FriendFilter may be in beta, but the accompanying TwittFilter is very much alpha. The link you have goes to the signup page, which gives no indication of this status, but the resulting place to login is a largely broken page with links that aren't functional and "alpha" all over it. It isn't very clear what is expected when logging in, and I got a number of conflicting status/error messages when it tried to create my file. Still not sure if it worked or how to confirm, which has me a bit concerned.

Topify (http://www.topifybeta.com/) is another entry in this field and looks like an even match for Twimailer, though I haven't checked that one out. I may try FriendFilter instead, though, because of the amount of checking it does for relevance and network overlap.

Are there any links to this data?