I prefer Twitter, Facebook, Pinterest, Google+, StumbleUpon, LinkedIn for the marketing works. Experts says we can also prefer Tumblr, its such a kind of Micro-Blogging platform but we can use as a social networking for our marketing needs. Definitely after the social giants Tumblr is the best prefer for marketing.
I have a lot of account in Social sites, but I focus on facebook, I will try to use twitter. it depend on the country which you market
I guess, Linkedin is a great place to extract most from social networking. But there are lots of things that you need to do make your presence effective on Linked in. Everything starting from selection of your photo, your profile completion, whom you are getting connected to counts a lot for achieving the best result on Linkedin. The most active promotion strategy on Linkedin is joining related groups and you should be active in responding to whatever happening there. That is enough to draw attention of others towards you and you can extract the most of it.