While I totally agree that search engine optimization should be considered as the number one way of getting traffic, the power of word of mouth shouldn't be ignored, especially if you have a product/site that "everybody" likes. I'll use the example of webcomics - sure, there are millions of em out there. One of my best friends runs one that is constantly ranked in the top 100, and most of his traffic came from word of mouth.
Basically, one person saw his site, thought, "Hey, this is funny," and told two friends. He guesses that, while he does active
SEO, most of his traffic (and recurring fans) come from word of mouth.