From: http://ycombinator.com/rfs3.html

Twitter is important because it’s a new protocol. Fundamentally it’s a messaging protocol where you don’t specify the recipients. It’s really more of a discovery than an invention; that square was always there in the periodic table of protocols, but no one had quite hit it squarely.

We could try to draw a small excerpt of this 'periodic table':

              LONG    SHORT
  ONE:ONE     email   im/sms
  ONE:PUBLIC  blogs   twitter

Of course, the axes aren't exact. You can write arbitrarily short emails and blogposts, and lengthy IMs. Twitter is usually 'public' but only viewed by some set of friends/acquaintances/fans, and interactions range from chatty conversations to long lagged correspondences. Each box bleeds into the others with crossover communications.

But you get the idea; the signature modes of each big success are variations on a theme, nailing a new permutation. And it shouldn't be surprising that blogging pioneers happened upon the adjacent twitter opportunity at the right time.

You could build similar tables where an axis-of-contrast is EPHEMERAL/ARCHIVED, or BUSINESS/PLEASURE, or TEXT/AUDIOVISUAL, or REALTIME/TIME-LAGGED, or SIMPLE/POWERFUL, and see some of the same things appear in the quadrants, or other familiar services, or gaping holes -- where there may be Twitter-sized opportunity waiting.