Twitter is the best for getting your business/blog known by people who want to do business offline (or at least off-Twitter.) It's great for any company OTHER THAN one that only sells their wares/services online. (Ex. NOT best for selling your ebooks, a t-shirt company, etc...)
Facebook is the best for chatting up your buds, following a company you may never want to buy anything from, or having a social media page because everyone else has a social media page. Most people here won't go from online to offline just to shop at your store or patronize your business/blog. Facebookers tend to stay on Facebook. Great for that ebook seller or t-shirt seller via page tabs.
Google Plus is the best for marketing yourself. That's it. From there, you can sway the masses to wherever as long as they like YOU.