The gist: the author (Gregor Hochmuth) argues that Twitter is still standing today -- despite its many well-publicized outages of late -- because it gives its users a strong sense of audience. (Hochmuth capitalizes "Audience" in a fairly pretentious manner that I will not emulate.) Audience, he points out, is more than just a friends list. To get the full Twitter effect, you need to have a clear sense of who you're writing for (rather than the blogging experience, in which content is thrown out into the ether in the hopes that someone will read it). To solidify that experience, the service needs to confirm that the people you're writing for are present, and that they're reading what you wrote. That functionality adds a sort of intimacy to what otherwise would be an impersonal, alienated experience. You send tweets to specific recipients, and you know from their @replies that they heard you. It is, in short, a conversation.
There's more good stuff there (for instance, why you should provide your members with more than one way to give feedback to their friends), but for me this was the core message. And it's an important one, easy to forget. I can say from personal experience that one of the biggest challenges in blogging is getting past the sense that I don't know who's reading what I write. I can go into Google Analytics and get raw numbers, and every now and then I get a note from someone I know who read a post, but for the most part I'm writing up articles and tossing them into the void. It can be a little disconcerting, as if I'm performing on stage at a venue where the lights have been turned so far down that I can't see who (if anyone) is watching.
This provides some insight into a fact that has always bothered me a little: that forums are so popular. I mean, sure -- I've spent my time on Usenet and in other discussion forums. I am aware of the initial appeal. But as someone who spends his day developing editorial content for websites, it's always a little off-putting when I fire up Omniture and find out that various forum threads are far more popular than my carefully-crafted content. But in the context of this article, it makes sense that forums would be so addictive. You post to a forum, and other people post responses. Immediately you get feedback that other people are out there, and they're reading your stuff. If you're lucky, they like your stuff and tell you how much they like it. That's a potent experience.
My takeaway from this: don't merely provide users with a blog or some other means of throwing themselves out there. Give other people a way to (immediately) respond, and give the original user tools to measure how much of a response (s)he's getting (and from whom). Build out an environment in which members are not merely acting, they're acting to an audience whose dimensions they fully understand. That's the killer app.