how to promote a website?

by promote i dont mean seo. i mean like how do you get people to use your website?
lets say it was 1995 and i had just built ebay. how do i convince buyers to buy there, and sellers to sell there?
or lets say i just made youtube. how do you get people to come to the site?
is it mostly word of mouth?

put a bunch of porn on it ;p

content. when i built a forum last year, it took alot of content i had to create. almost felt like talking to myself for alittle while. 6 months later and kicking at 150 members and steady post count.

depending on you site, get interesting stuff, content, and make it appealing.