The biggest mistake marketing and brand managers make when approaching social media is not thinking of the social web in the same strategic light as everything else they produce.
Social media is no more a one-off playground for brands than television advertising, direct mail campaigns or customer relationship management programs. It's serious business and should be treated as such.
The following question then becomes relevant: "How do we think about social media strategically?" The good news is that it's not that difficult, provided your