Where do we begin?
Every time I write a post, I have a dilemma.
It matters, of course, because I can take shortcuts, it changes the perception of my tone of voice and I can skip a lot of the preliminaries.
Or am I writing for you, the first-timer, the person who found this post on Digg or Delicious? If it's you, then I should take my time, write a bit more, put some background links in, etc.
Now, of course, you have the same dilemma too.
You have it when someone friends you on Facebook. Maybe they found you cause you're cute, or because you just joined a new company or because you're a friend of Tom's. Or maybe they've known you since summer camp and you just need to reconnect...
I think this dichotomy of experience raises the level of responsibility for the reader. Without knowing who you're reading, it's hard to judge the tone of voice of what you're hearing. More important, it changes the posture of the writer.
Sometimes, the web is more of a cocktail party than a club meeting.