Here is my experience.
I have posted my merchandise on many sites. If you are lucky to get posted on a well read article, like a LOOKBOOK for fashion or a jewelry or home magazine if that's you product, fantastic. This will bring you a lot of business. It not it has been recommended that you post on tumblr or Pinterest. From my personal experience, I have not received any business from either one of those sites.
I definitely do believe that making a store twitter and Facebook account are important to support whatever other social site you are using. But don't just rely on the autobots to post for you. Show up in person and leave a little bit of yourself there. Say hi to your friends. Share yourself, but don't share too much. This is a tricky subject and best left to another post. Just make yourself real and likable.
As for where to post? I am positive that it is different for different types of merchandise. I find that I get the most direct sales from Wanelo. I can see direct correlation from my visits to my sells. I actually get a lot of visits from google shopping, but those rarely make sales. Wanelo makes sales.
I also get a LOT of visits to my shop from RebelMouse. It's a nice little social network that just reposts sharing tweets or facebook posts. Seems to bring people into the shop.
I also get a lot of visits from my website and from blogs I have ads on. If you have a website, make absolutely sure that you are using your metatags to show what you sell on your shop so that people can find you in a search. Also make sure to occasionally post your website socially, even if you don't sell directly off of it. Any posting you do will rank you up on search engines. The more visits you get, the higher your ranking. Remember, your website is there should you decide to switch selling venues. People will know where to find you.
Remember to give Wanelo and RebelMouse a try. You'll like them I think.