I know this question has a lot of possible answers, so I'll break it down to several questions so you'll understand exactly what I want to know.
You should register them all, so that nobody else can masquerade as you.
Whether social media is useful to your business depends on the context.
Simple answer: You should have the appropriate social media accounts that will enable your company to communicate with your fans/users/customers.
We're a growing web startup business, and we've been making a big push on social media this year since I arrived at the company. From what we've experienced, Facebook is by far the strongest engagement tool for your users. It links you to a vast web of your followers their friends, and friends of friends - a great way to grow organically. Twitter is good as you can interact directly with individual users; while we're exploring how a well made Pinterest account can drive new traffic to our site currently.
BUT the key point is that NONE of these will work unless you truly put time and effort into building a truly engaging profile on each, so make sure you have someone who knows what they're doing, because a half-hearted approach to social media could even be damaging to your brand. Of course you can simply learn from what similar businesses have tried on their social profiles, and then copy them!