He's not taking any business away from Twitter; if anything, he's acting more like a Twitter affiliate and generating press for the company, and he states clearly that his website is unaffiliated with Twitter.
Scenarios:
* John Chow's site becomes big and a journalist writes an article about JC's site in the New York Times. Thousands flock to JC's site, of which a few dozen join Twitter.
* John Chow optimizes his website for social network-oriented keywords such as "social network marketing" or "advertise to millions". His site appears on Google for those keywords. People Google those keywords and discover Twitter through John Chow's site.
...etc.
Trademarks were created primarily to help prevent business from being taken away from IP owners by confusing consumers. JC is, in a sense, doing the opposite.
Plus, Yofie is correct that Twitter doesn't have much of a history of defending its trademark anyway.