Beyond the hype, Twitter is significant because you can crawl their complete social network graph of millions of users. Delicious et al offers a similar crawling but they don't have the same user base, and facebook doesn't permit to do this.

Why it's important? from a business perspective you can quickly analyze the customer sector of companies: look at dell followers, compare it to the apple ones, what's the difference? (don't know if apple is on Twitter so take this as an example)