Ben Barren says something very smart on Twitter, that bears repeating and amplifying.
The podcasting boot-up in 2004 happened much faster than the blogging bootstrap for one simple reason. We already had blogging to carry the message. "Hey there's this new thing you should try!"
Remember that when thinking about how high a barrier to entry the existing social silos have. How can they stop the message of their successor from propogating? Maybe they can. Better start thinking about it Dick and Zuck. :-)