promoting web 2.0 sites

suppose a web 2.0 startup has completed its product and is trying to launch it...what are some of the most effective ways to promote the product that is specific to a web 2.0 model (ie. users come in to contribute to the content) rather than say generic methods such as SEO, link building etc that are applicable to more generic sites such as blogs

more specifically, what should the web 2.0 site be doing and what are some of the important strategies to execute in order to induce users to contribute content as well as to generate more traffic?

Much of it depends on the niche, but here are a few ways that may help:

• Be social in your space on Twitter, Facebook, etc.
• Do white hat SEO and write great content which would get people to land on your site's pages with relevant searches. Also, don't link-build, but focus on making great product. More links will come after you are link-worthy rather than a link-begger.
• If you can afford it, SEM tends to have a great conversion rate if you do it well.
• Try to get PR (but only after your product is good enough)
• In general, your users will do much of the promotion for you if your product is good. If they are not doing your promotion for you, you should think about why not. It might give you clues on the quality of your product and where you can maybe improve.

If users are going to contribute and create your content/value of your site, then they have areas of specific interest.

Find out or guess what they are... and go participate in like minded communities to encourage them to participate in yours.

Example, you've got a photo sharing website. Go to photography forums, blogs etc make relevant comments and contributions and promote your service where your users already hang out.