On the surface, social media and knowledge management (KM) seem very similar. Both involve people using technology to access information. Both require individuals to create information intended for sharing. Both profess to support collaboration.
But there's a big difference.
Knowledge management is what company management tells me I need to know, based on what they think is important.
Social media is how my peers show me what they think is important, based on their experience and in a way that I can judge for myself.
These definitions may sound harsh, and biased in favor of social media, and to some extent they are. Knowledge should be like water — free-flowing and permeating down and across your organization filling the cracks, floating good ideas to the top and lifting all boats.