If you google a few terms like Sony Viral Video Backlash you’ll find out what happens when marketers try to use the participants of a social network, instead of just trying to harness the network.
I once organized a charity Walk-a-Thon on Facebook, and I got to see what happens when you use a network, instead of its participants.
What’s the difference? Transparency. Participation implies consent, and when marketers try to trick people into doing their advertising for them, bad things ensue.