I suspect that you didn't read the article. In addition to heavily discussing Dunbar's number, the entire point of the article is that, even with the aid of social networking technology like Twitter, Dunbar's number still applies. From the article:

"The bottom line is this: social networking allows us to vastly increase the number of individual we can connect with. But it does nothing to change our capability to socialise. However hard we try, we cannot maintain close links with more than about 150 buddies."