It's also worth noting that it can actually hurt your image if you try to bolt on virality when you don't know what you're doing. You can become a laughing stock. I'm reminded of recent failures by two of the largest engineering companies in the world.
Microsoft: http://www.istartedsomething.com/20090917/windows-7-now-with...
Siemens: http://www.lc-d-825.com/