It relies heavily on word of mouth advertising; someone sees or reads what you
have to offer and tells a couple of their friends. Their friends see or read your
message and they tell a couple of their friends, and they tell a couple of their
friends, and so on. The number of people who are aware of you and your
message increases exponentially. The more people hear about your site, the
faster your traffic grows. How large will it grow? There is really no way to tell.
The only real limit to the amount of traffic possible is how many people your site
host can handle.
Just think about how quickly it could spread. If one person saw your message
and told 2 people, who told 2 people, who told 2 people, and it continued, your
message would very quickly be in front of thousands of people. If each person
just told two other people, look how quickly it can grow.