One of the most effective forms of promotion is word of mouth.
People often tell their friends if they have found a great site.
So if one person tells ten of their friends about a site and then one of these friends tells ten other friends and so on and so forth. As you can see the possibility exists for these things to snowball around the planet. A good example is the now famous hamster dance site. It became so popular primarily through this type of phenomenon. There are several options by which you can make this process easier for your visitors.