I have met quite a few companies that just plunge straight into the social pond without any purpose or plan. There is nothing wrong with that since you will at least learn how the different platforms work and how people interact on them.
But, if you really want to leverage the social web you should start by asking yourself these two questions:
1. Why do people buy my products and what kind of value do I bring to market that differentiates me from my competitors?
2. Who do I really want to connect with?
By asking and answering to these two questions you will have a good starting point for building your social media strategy.