I have a Pinterest, Twitter, Tumblr, Facebook, Wanelo and a Stumbleupon.
Seriously to post to all of them daily only takes about an hour's time. It may seem like a lot, but I think the more exposure I can get the better. Most of the sites allow you to link posts to other sites. For example, in posting to Tumblr, I can automatically send the post to Twitter. I don't get how twitter is an effective marketing tool, but I have one and I post from tumblr and randomly on things I come across on the web.
Like someone else said earlier, Tumblr is mostly younger people browsing and not really shopping, but they do reblog like wildfire which allows for more exposure. I don't spam and bombard with just products and promotions, but reblog fitting fashion photos, etc. I do post photos of my products and a link to my store. Some days I have upwards of 500 views just from Tumblr alone in my Etsy shop.
I try to post to Pinterest daily from my Etsy shop, especially for new items or renewed listings.
Wanelo: is sort of like Pinterest and I've been posting more to that site lately.
Facebook: I share relative articles about art or jewelry, along with a daily featured listing from my shop. I am a photographer also, so I make my own inspirational quote art and post that as well. I also have a monthly "like" and "share' contests which get some traffic and new likes. I have taken advantage of the paid promotions that Facebook has with a little bit of success. I'm not sure if I got any sales out of it, but I have gotten page likes...which builds my street cred. lol. I really get aggravated that Facebook hides your posts from some readers in an attempt to get you to pay for post promotions, and I almost want to just do away with it. As we all know the whole world is on Facebook, so from a business standpoint it probably is best to keep the page active.
I have just recently started using Stumbleupon and I get a little bit of traffic from it. I am still learning how to optimize it.
I hope this was helpful. I am in NO way an expert on any of the social media outlets. I enjoyed reading everyone else's tips though. ;)