Blogs are great for getting people to a site. Twitter is great for tossing around short-form ideas and quips. Facebook is great for talking with a defined community.
But blogs are not inherently social. They try to be, with comments and RSS, but they're still built in silos. Twitter is unbelievably social, no doubt about that, but it's also shorthand. It's very hard to have an engaging conversation in 140 characters. And Facebook is like a ping-pong match: lots of back and forth excitement, but very little substance.
Buzz could be the missing link here.