I'm selling a handmade eco friendly product so these are the social networks that I've decided to work on:
Facebook: I'm using Facebook because it's HUGE (now) and I am very comfortable with it. I have around 1200 friends already on my personal page so I'm hoping that I can bring some of them over to my product page.
Etsy.com: This is not only a community but a place for a virtual store. They have respectable sales and traffic.
Pinterest.com: You have to get an invitation to use Pinterest which I just received so I haven't done anything on the site yet.
I think that any start up will take you in directions that you never imagined. Having a following early isn't just important for future sales, but for having a dedicated focus group that will help minimize the bumps along the way.