If you were setting up a new forum now, would the Diggs and Facebook of the world affect how you did it?
By nature, I believe, all social media, including forums, are complementary.
There are no either/or choices between; face-to-face talk, telephone call, email, instant message.
The nature of the message, the availability of others, and the hope, expectation or need that the message results in a conversation, will determine which medium to use when.
Taking a step outside the link-happy moments ("oh look! a rainbow!"), a forum by its very nature has a huge thing going for it: it is meant as a place to talk, discuss, converse.
I could start a conversation on any subject right now. I can't do so on Digg, Reddit or Sphinn. These require me to have something to point to (the URL) to talk about.
Only at a forum can you be the thread starter...