I think the popularity of apps like Twitter comes from a slowly-growing acknowledgment that people want to publish different kinds of information / have different levels of communication, and that specialized tools can leverage this want. In other words, sometimes you use Blogger / Wordpress, sometimes using Tumblr or whatever makes more sense, sometimes Twitter, sometimes using a shared-bookmarking site w/annotations, sometimes just posting a comment somewhere, sometimes sending an email, text message, IM, etc; all of these are specialized towards different levels of communication and different use cases, and it's interesting IMO that instead of convergence we're seeing the emergence of discrete apps and then applications which (or APIs which make it easy to) aggregate these divergent modes of communication (ie FriendFeed, etc).
