The domain was Created: 1997-09-11 and according to the waybackmachine the first record for the site was Dec 02, 1998 and it was a much smaller classified site until June of 2000. While they did add more categories around this time they were only serving the San Fran area. A short time later, according to the WBM sometime in August of 2000 they began adding other cities and even a couple in Australia. Now they may have been serving other areas before this but I am getting this info from the Way Back Machine so it isn't going to be all inclusive.
As far as good
SEO being behind the site's popularity - that isn't the case as some quick checks reveal that the relevancy numbers are quite low. The site was more than likely one of the first social networking sites out and on the Internet being first at something can give you a huge advantage. Another possible reason the site did well early on probably has to do with their Personals section. If enough people get lucky while using the system well then you know that the word is going to spread faster than a hooker's legs when Charlie Sheen walks into a brothel. (Sorry Chuck I couldn't help myself)
Lastly - the site is too easy to use and it is completely free and remains so without a bunch of schlocky ads gumming up the works.