I've always wondered this. Twitter is a great site to be on for marketers to gather and share information, so I've wondered why one of the most notable marketers was missing from the site.. Well, Seth Godin answered this question in his blog today:
If you have a presence on twitter, squidoo, blogs, facebook, myspace, linkedin and 20 other sites, the chances of finding critical mass at any of them is close to zero. But if you dominate, if you're the goto person, the king of your hill, magical things happen. One follower in each of twenty places is worthless. Twenty connected followers in one place is a tribe. It's the foundation for building something that matters.
This is why I don't have a podcast, a video channel, any activity to speak of on Facebook. It's why I don't use Twitter or travel the country visiting bookstores. There are many places to be, and it's tempting to act like those non-profits and race after the next one. But it doesn't work.
Basically, since he can't devote enough time to be a heavy user on Twitter, he is staying off the site. His recommendation is: if you can't dominate a site, stay off it. Don't spread yourself thin with 20 different posts among 20 different sites. Only join networks where you can invest time into. What do you think?