They do know, I have a friend who works at a seeding agency, and they know very, very well how it works.

Make it funny/stupid/sexy enough, and people will watch it.

A couple of good examples of this:

http://www.youtube.com/watch?v=6xfBNxNds0Q http://www.youtube.com/watch?v=owGykVbfgUE http://www.youtube.com/watch?v=USYa5iH3f3s http://www.youtube.com/watch?v=JJmqCKtJnxM

Don't know if the latter was launched as a viral campaign, but my point is that you know it and the brand it was from if I'd phone you up and shout "WASSUP".

Marketing is a lot scarier that you think.