ShadyEmail has a component of that, it encourages you to email your friends with a shady address. But by and large I get much more traffic from google than through random twitter links, etc (Of course this could just mean that i need to spend more time on this type of social marketing).
Viral-marketing & social marketing is still just marketing, i've been behind the scenes on a few campaigns that "went viral" and though they fail readily if you don't have the content to support them, even good content needs to be driven to critical mass before it catches on. Even then it dies out quickly without someone pushing the iniative.
I don't think any one approach is right or wrong, but for me Facebook+Twitter drive 3-4x more traffic to my site than Google. Getting good google "juice" is important, but not as important as it was a few years ago.
Viral-marketing & social marketing is still just marketing, i've been behind the scenes on a few campaigns that "went viral" and though they fail readily if you don't have the content to support them, even good content needs to be driven to critical mass before it catches on. Even then it dies out quickly without someone pushing the iniative.