How do you build an audience for a blog search engine? You can be good, you could even be better than the most popular engine. But what it really takes is smart business relationships. Reading Steve Rubels blog via Bloglines, means that you are reading his feed served by FeedBurner, which means that Steve can add FeedFlare elements to the feed, which means that you could hit the handy 'Related Posts from Sphere' button to see, well, related posts served from the Sphere site.
You can't just be good, you have to spend effort (which means time and money) on relationships that don't have a clear input and output (where the output is of type MONEY). Technorati was always very good at that, and stealing Technorati's position means building these relationships to ultimately create a portal that is synonymous with blog search.