In the 1500s Sir Thomas Gresham observed that Cheap money drives out dear. This originally applied to two types of money, e.g. gold coins and cheaper metal coins. The reasoning is that people with two types of money will hand over the bad or cheap ones, and hold onto the dear ones. The law was actually named after him by another British economist in 1858, and has been applied to many areas where the true value is different than what people need to accept.
Twitter has triggered a similar tendency on the internet. Cheap comments (140 character tweets) are driving out dear comments (multiple paragraph blog posts). I'm a very guilty party. In the last few months I've done dozens of tweets and fewer than a post a month to this blog.
The danger is that people with serious thoughts about a subject, which would call for a several paragraph thoughtful post, properly cross linked to interesting web content, will now just tweet the essence. And the dialogues that great blog posts engender will be lost.
I'm not sure there's anyway to stop this (after all, it is the law), but I'm going to try and add more blogging to my twittering.