Twitter, For the Win
You can honestly do well with social media using just Facebook and Twitter alone. But Twitter is great for a number of reasons.
1) Distribution of your content - Twitter serves as a great way to send your valuable, targeted content to your valuable, targeted audience.
2) Crowdsourced content ideas - you can use tools to glean valuable focused information about what your questions your target audience has and then answer those questions with your content. Similarly, you can see what interests them, their pain points, etc. and create content that speaks to that.
3) Networking - when done right you can meet some amazing people on Twitter. Relationships and even partnerships can be forged on Twitter.
4) Recruiting - If you want to find some top notch talent out there, you can definitely do so on Twitter.
5) Ask questions - Need help or direction on a project or content creation idea? just ask Twitter. Want to get some feedback on a project idea? Tweet it out. The Twitter community is great for this once you are able to sift through the noise.
Hope this helps,