It really depends on your Target audience. Are you trying to sell something visual? Do you want to get professionals involved? For starters, Facebook is a must! Why? this is where everyone “hangs out online”…You must be on Facebook and have a prescense. If you want to sell clothes/jewelry…Pinterest and Instagram should be your staples.
If you want to network with professionals..Google+ and Linkedin are best.
Wherever you go, just remember to STAY SOCIAL. It is called “social media”, do this by engaging with others, asking questions, listening to what your followers have to say and so on. Its funny how so many people forget about the Staying Social part of Social Media and just sell, sell, sell.