There are countless other social media websites online that cater to specific niche audiences or even larger general audiences. Clearly there are only a handful of “big dogs” online right now who appear to be leading the pack. This does not mean that you should ignore the others, nor does it mean that you should spread yourself too thin by trying to be all things to all people.
Let your customers and your prospects guide your decision making in regards to which social media sites you will hang out on and participate in. If there are other social sites that you find to be beneficial for your own self-improvement or advancement, of course use those as well.
However, one of the most basic lessons is that you should be where your audience is. Right now the odds are your social audience is on Facebook and, if any other second choice is an option, then it's probably Twitter. Start on those two for now. Others worth checking out include linkedin.com for professionals, and FourSquare.com if you have a business location that receives walk-in customers.
What are your thoughts on social marketing? How is it working for you? Are there other major sites that should be included? Stop by the book resource page and let us know what you think.