All my life, I've been "David," except to my older sister who calls me "Dave" or even "Davey."
If you call me "Dave," I won't correct you, although if you ask me my preference, I'll say "David" without hesitation. If you ask me why, I won't be able to give you a meaningful answer other than that my family called me "David."
Now, at age 53, I find I'm becoming a Dave. About half the time.
The explanation is, I think, simple. These days, most of the people I meet aren't introduced to me by someone who — one or two or six degrees ago — I introduced myself to as "David." Because we meet via the Net, these new friends and acquaintances have to take a guess, and "Dave" sounds less formal than "David." So, "Dave" it is. And since I don't correct them (see paragraph 2), "Dave" has begun reinforcing itself.
I'm guessing that this doesn't happen as much in the world of print publication. If I were to write to John Updike, I wouldn't start the message off, "Hey, Johnny!," even if I were sending email. Likewise, I doubt readers wrote to Ernie Hemmingway, Jackie Steinbeck, or Aggie Christie.
But, much Web writing feels so immediate, so personal, that even though the architecture of the relationship is one-to-many, and thus is formally like the broadcast architecture, it's more like the one-to-many at a party where a group of us are telling stories, giving each other the floor.
Furthermore, for much of Web writing, especially blogs, the distance between the author and the work is erased. We are who we write. In responding to my Web writing, you're responding not to the work but to me. I suspect that some people call me "Dave" precisely to announce that they're talking to me, not to an author of something. "Dave" drives a wedge between the by-line and the person.
(By the way, I still prefer "David.")