The secret of Viral Marketing (VM) is that it’s all about popularity. Which is to say … sooner or later, it’s all about sex.
When we made Perplex City, the Alternate Reality Game, I learnt a lot of basics about VM just from going out there and trying so much new stuff on a week-to-week basis. It was a small startup, so (at least at first) everyone got involved in everything, and you got to see the things that crashed and burned in as much detail as the ones that shone like gold (this was before the company grew enough that people started covering-up the failures).