Viral Marketing: Engineered or Happenstance?

I had a fascinating conversation yesterday with an entrepreneur who has run a very high growth peer to peer business and has started a new venture which is, in large part, looking to grow through viral marketing.

We debated at some length whether sites who enjoy rapid growth through viral marketing do so as a result of cleverly engineered features and strategy, or through good timing, good content, and/or good luck.

I am sure the answer is to some degree both, but I have now encountered plenty of folks who argue one or the other.

What do you think?

Please comment!!!