I saw nothing but a picture of an RSS icon, and thought that was the joke: you can "follow" people by subscribing to their RSS feeds, and RSS already exists.
Twitter became popular because browsers didn't elegantly handle feeds. Tumblr blogs (or blogs in general) with post comments and following are much more open and just as usable as the Twitter platform, if you provide easy to use rss syndication and browsers support easy to use feed browsing without having to search out a google reader replacement.
