What is the biggest challenge of social media?
Stepping back from the marketing and social media question for a moment and thinking about the spread of information in general then the rapid diffusion and adoption of ideas has to be one of the greatest benefits of social media.
However, the distribution of 'misinformation packaged as fact', or what Damian Thompson defines as Counterknowledge in the book of the same name, may be one of the biggest downsides. In the past pseudo-history, quack medicine, and bogus science lived among the "cultic milieu" at the edge of society where it was tolerated and contained because the structure of society worked to weed out the riffraff. However, with the removal of hierarchy in our daily lives (school, church, jobs, government) we no longer rely on others to tell us what to believe but adopt ideas as part of our own brand. Thus the burden of proof, if one is even required anymore, now falls on the individual.
Social media, with penetration getting well beyond any tipping point, makes the transfer of ideas both meteoric, morphable and unfortunately too often presumed as 'true'. The result is the increased repeating of ideas and opinions based on facts that aren't facts. Example: pretty much any negative political campaign leverages counterknowledge.
If for no other reason than avoiding unwanted associations, marketers should listen to what now crosses easily into the mainstream.