To answer the question "why should I use Twitter / Facebook / Flickr / Digg / Sphinn / Jaiku / <insert Web 2.0-ish site>?" you have to rephrase it:

Why should I settle for less attention?

attention

Having a blog is just the start.

"Your" audience isn't hanging on to your every word: they're leading their own online lives.

The only way to catch their attention is to be where they are.

The more places you and your audience share, the more of their attention you have.

you-and-audience

It's the participation economy, stupid!

Recommended follow-up: