The "tipping point" theory popularized by Malcolm Gladwell's book was proven to be too simplistic and flawed by Duncan Watts, a network researcher currently at Yahoo. His book "Everything is Obvious - Once You Know the Answer" debunks this popular misconception. He describes many studies, simulations and actual experiments that show that how fast something spreads virally has less to do with where it starts (influential groups/tipping point), than how susceptible a person is to being influenced (ie how infectious the idea is to begin with). The book is great btw.