I agree with what Alek_MochaHost mentioned. Trying to explain how to use each social media site to your advantage will take several posts, and its not easy to mention them all in a post.
But if I were to try, here's how I would describe these social media platforms:
1) Facebook: The most generic site with the most number of users, which makes it the best one to start with. There are a variety of ways to interact with your followers, which makes it really flexible and open to your imagination.
2) Twitter: More suited for quick and direct engagement. Many companies have now used Twitter for CRM, so you might want to invest time in Twitter if your business is going to involve much customer service.
3) Linkedin: Targets business professionals, which makes it a good platform if your products serve corporate needs (e.g. your business is B2B). However, if you are targeting mainstream consumers, it wouldn't really help to be using LinkedIn.
4) Google+: A new player in the social media scene but is worth your investment if you are looking to improve your SEO ranking. The introduction of Google Authorship makes Google+ profiles ever more important. You can even utilise Google Hangouts as a form of customer service, or consumer engagement strategy.
Ultimately, there isn't a need to use ALL of these social media sites, but to focus on the appropriate ones. I would suggest you to invest some time in Facebook and Twitter first, and if your company is a B2B business, you should invest some time in LinkedIn.
Hope that helps!