[S]ome ideas are just bigger than 140 characters. In fact, most good ideas are. More importantly, our ideas often need to gain traction and meaning over time. Blog posts often age into something more substantial than they are at their conception, through the weight of time and perspective and response.
And blogs afford that sort of maturation of an idea uniquely well amongst online media, due to their use of the permalink (permanent link), which gives each idea a place to live and thrive. While Facebook and Twitter nominally provide permalinks as well, the truth is that individual ideas in those flow-based media don't have enough substance for a meaningful conversation to accrete around them.
Dash also points out the biggest problem with Twitter at this point: there are no publicly accessible archives. There's no easy way to search your Twitter stream or the streams of others. This means you should fully expect anything you say on Twitter, no matter how important or profound, to be completely inaccessible and lost to the ages, unless you do something to preserve it. The fact that most people ignore these limitations makes it all the more tragic that great conversations and great ideas may never be read by people who just weren't following their tweets at the time.