There is nothing exclusive about twitter & blogging. You'll note those with the biggest personal brands and most followers are often bloggers. This is because facebook and twitter are both bad at content/person discovery.
Blogging is a good way to let people find out who you are.
I'm not a fan of people who use social networking to advertise. I know there are practical benefits, but the idealist in me wishes that people wouldn't pick up media just to abuse it. Some things are great: I remember there was a comment here about somebody who complained about Comcast on Twitter and got an instant response. But I've seen too many people who use Twitter and Jaiku and Tumblr to post links back to their content and that's that, and I hate that.
Of course, the idealist in me thinks that you shouldn't advertise in every venue possible, that you should work until you've got something so useful that word-of-mouth brings people in, and that's not entirely practical either.
What if you have people come to you instead of vice versa? For example, you never follow anyone on twitter you make people follow you. I agree that the spammers are a huge problem, but I think some of these tools can be used for good.
I raged about Comcast on Twitter yesterday and they haven't done anything. Bastards.