read this article on techcrunch, some interesting points
Imagine, if you will, that the entire Internet is contained within a single continent. That continent is filled with countries, states and cities. Each jurisdiction is autonomous, relying on visitors to cross on to their turf to engage in commerce. Now, imagine if the only way to get into this continent involved just two methods: SEO and SEM. Let?s further imagine that the borders to this continent were controlled by a single company. Let?s also hold that the rules for search engine optimization listings and search engine marketing were not only defined but were completely controlled at the whim of this single company. Of course, we all realize that word-of-mouth marketing and viral marketing also contribute to traffic to individual websites. That said, the primary methodology for all users to reach any individual website destination is still search, of either paid or organic listings.
Or suppose the paradigm is the streets of Los Angeles. Let?s imagine that in order to enter the city you had to pass through a single gate. And once you entered that gate, the streets you were or were not allowed to go down ? and thus the businesses you were or were not allowed to visit ? could be randomly blocked from your access. Blocked to a point where you might not even know they exist; whatever streets were available for you to traverse were in essence the only streets you knew where business could be transacted.
http://www.techcrunch.com/2009/07/13...eting-and-seo/