An audience is a group of people brought together by a common interest.
The difference between an audience and a community is relationships. Community members have relationships with one another, audience's don't.
On social media channels you build audiences, the focus is on disseminating information. In communities the focus is on facilitating interactions between members.
The benefits of building communities are high. You can increase spending from existing customers through upselling, increase in repeat purchases, higher levels of retention. You can attract new customers and improve lead conversion %. You can reduce marketing/customer service costs.
For organizations that have built big audiences on social media platforms, the next step should be to turn this audience into a community. Yet, at the moment, this rarely happens. Too many stop at building a Facebook page or Twitter following and fail to gain any measurable benefit.
We've been working with Ning on an exciting project to change this. We're creating a place where everyone can turn their social media audiences into communities. We're helping to develop a platform which most people can afford, is in-line with best practices (from social sciences), and is simple to use. I recommend you take a look.
Regardless of your platform, building an audience should not be the end goal. Moving from communities to audiences is to move backwards. Push towards cultivating genuine communities on community platforms. It's far easier to build a community when you have an existing audience, make sure you're making the most of it.