It really is remarkable, but there are a lot of other forms of communication still working, too: phones (including land lines!), copy machines (seriously underappreciated for their role in breaking down Soviet thought control), personal communication.
Twitter, though, is visible to the rest of the world, and much harder to filter quickly than blogs. Still, I’m actually concerned, a bit, about this: it leaves a record, one that’s very difficult to erase, and if the regime regains control, there will be a vicious backlash against identifiable twitterers.
Update: Apparently I'm not wrong.