I just listened to a podcast of CBC's Spark with Nora Young, and it started out with the most interesting story about an experiment by a Danish university professor. Anders Colding-Jørgensen created a real Facebook group about a fictitious cause: “No to the demolition of the Stork Fountain“
Although there was no threat of the Copenhagen landmark actually being destroyed, a fact which he referenced in the discussion page, the group was joined by thousands. Even though some people realized what was happening, they were powerless to stop it. Someone posted on the wall "Do not join this group - it is a Hoax!", and even included a screen shot of the disclaimer in the photo section of the group, but it continued to grow to a membership of over 20,000 people.
When asked why they joined the group, most people said that the fact that the group was so large gave it credibility. They risked nothing by joining and had nothing at stake, so they didn't bother doing any research into the issue or opposing views - They just joined and moved on.
I'm not really sure how much credibility social media groups have in the first place, but this experiment shows we should give them even less credence.