Forbes.com has a short but informative article in which Roelof Botha (the Sequoia Capital venture capitalist instrumental in the success of YouTube and PayPal) shares two tips on how to have your service explode in membership using viral marketing.

Botha points out that "word of mouth" and "viral" are in fact distinct ideas, while most entrepreneurs consider them interchangeable.

Word of mouth is when I tell you to shop on Zappos because I think the service is great," explains Botha. "It becomes viral when you have to be ‘in the system’ to use it. For example I can post a video on YouTube but then you would need to go to the site in order to see it.

If you have a Web 2.0 startup that you're trying to spread through viral means, take a few minutes and read this article.

A Gold-Plated VC's Billion-Dollar Secrets [Forbes.com]