Designers come from a tradition of figuring out the whole system, and putting it out there. But the success of Twitter has emerged from an approach that’s nearly 180 degrees from that. Twitter began simply as a way to post 140-character messages, and allowed remarkable freedom in following. Almost no structure was placed on the system. It’s through the paths that users have worn in the system (e.g., @ replies, retweets, hashtags, followfridays) that Twitter has grown to realize the value of the service, and they’ve made initial steps to “pave” those most popular paths (most notably replies, though Doug mentioned that they are considering how to more formally support retweeting as well).