There have been studies on this that show it only takes a very small number of users to create a critical mass and steer social media discourse. The effect is amplified by being early in the discussion (ie; if you can control the first few comments, you can control the position the rest of the users take). The researchers would start identical topics on sites like Reddit and manipulate the first few comments to take opposite positions and then analyze the overall positive or negative impressions in the thread.
I believe the studies were mostly focusing on political topics, but PR and Advertising are closely related (see
Edward Bernays) and it didn't take long for companies to start taking advantage of this to sell product. A tuned-in group that can get the jump on the narrative has a huge impact on what follows, this is how a dead forum with a few hundred active users becomes a powerful voice.
80 percent of the content on Twitter is generated by somewhere around 10-20 percent of the users. If you target the top 5 percent of users you can get a large amount of positive "grassroots" publicity at a very low cost (these motherfuckers will suck your dick for a free poster or DVD every couple of months), the amount of money that traditional advertising and promotion is fucking mind-boggling, social-media is a wet dream for this industry, in their wildest dreams, they would never have expected to get so much positive hype for so little cash.
This is how we wind up with people like T. Greg Doucette making totally ridiculous and indefensible positions on Twitter etcetera for months on end.