On the bottom floor? You must be kidding me. Average salaries of CS graduate after 3-4 years out of school are in six-figure range in most parts of the country. That's in top 5% of US population and you can hardly call that a "bottom floor": ask your mechanical or civil engineering friends how do they feel about such "bottom floor".
My younger brother was kind of pissed with his $80K offer from Microsoft immediately after graduation claiming that some of his friends did better. That's ridiculous.
Stop kidding yourself, huge percentage "Web 2.0 entrepreneurs" just aren't interested in building real value-providing businesses. Startups like Twitter aren't built to be companies, they're built to flip - it's included in their "business model". I can't speak for more well-known startups, but those I am personally familiar with are all built on the premise of M&A nearly immediately after starting to break even.
THAT is the primary reason why there aren't too many "next Googles". Not many people are ambitious enough to aim than high.
It is kind of sad that Paul rarely participates in discussions like this one, but understandable given his limited time, but I'd love to hear his opinion on this.