When it comes to search traffic, google is near monopoly numbers, especially if I measure search traffic with Google Analytics.
Here is one example:
I have a site that has not done any sort of offsite SEO in years, and got 57,000 hits from Google in the last 30 days, with Yahoo as the #2 at 2000, and Bing at #3 with 1700.
Our business model does not depend on Google because this is a non-profit, and the goal is not to make a lot of money, just to pay server expenses, etc. If we were running it as a business, we could send out a newsletter to 100,000 every couple of weeks, and get some decent traffic.
But if this were a business, and google decides to cut us off, it would be a big deal.
I think in time, challengers like Bing and Facebook will change the dynamics, but I don't really care for sites like Facebook that seek to own all the traffic themselves, and put a wall around parts of the net.
I would like to think that Google would not support this either, and would instead send more traffic to sites that remain open to all. A closed link structure changes the whole nature of the WWW.