The key concept is intimacy gradients. In a well-known passage from A Pattern Language the authors write,
The street cafe provides a unique setting, special to cities: a place where people can sit lazily, legitimately, be on view, and watch the world go by… Encourage local cafes to spring up in each neighborhood. Make them intimate places, with several rooms, open to a busy path, where people can sit with coffee or a drink and watch the world go by. Build the front of the cafe so that a set of tables stretch out of the cafe, right into the street.
That’s the passage as quoted in the book’s Wikipedia page. But if you actually look at that section of the book, you’ll see that the authors place a great deal of emphasis on the need for the ideal street café to create intimacy as well as public openness. Few people want always to “be on view”; some people almost never do. Therefore,
In addition to the terrace which is open to the street, the cafe contains several other spaces: with games, fire, soft chairs, newspapers…. This allows a variety of people to start using it, according to slightly different social styles.
And “When these conditions are present” — all of these conditions, the full appropriate range of intimacy gradients — “and the cafe takes hold, it offers something unique to the lives of the people who use it: it offers a setting for discussions of great spirit — talks, two-bit lectures, half-public, half-private learning, exchange of thought.”
Twitter actually has a pretty highly developed set of intimacy gradients: public and private accounts, replies that will be seen automatically only by the person you’re replying to and people who are connected to both of you, direct messages, and so on. Where it fails is in the provision of “intimate places”: smaller rooms where friends can talk without being interrupted. It gives you the absolute privacy of one-to-one conversations (DMs) and it gives you all that comes with “being on view” at a table that extends “right into the street,” where anyone who happens to go by can listen in or make comments; but, for public accounts anyway, not much in between.