It can be mind boggling, I think you need to look at your business and your market, for instance if your product is visual, then pinterest, facebook, instagram can be great. For example my good friends at Back to the Wall are having a lot of success with pinterest. Having such a visual, designer product has led to many overseas enquiries from customers who have seen their murals on pinterest and then have been repinned, again and again.
Or sometimes it makes sense to stop the guess work, and to contact a professional. I was introduced to Tom from Houdini Search Marketing the other day, so if you need some help with adwords, copywriting, social media. Look him up.