1- The site encourages the user to rely on it for social status, or enhance the 'cool' by association effect, and
2- promotion efforts for the site are high.
The best known ones, like MySpace, are already part of a much larger conglomerate (News Corp.), but savvy net users can begin tracking potential successes early on. Start off with a company or a web service that's gaining a lot of buzz; watch for news that the web service has secured VC funding, and you'll have a short list of prospective companies gearing up to go public.
