The fantastic research and graphics are from the Information is Beautiful blog.
If this is applied to your Twitter followers, what's the true percentage of your target audience that wants to hear from you and engage with you? It's likely a small percentage of your overall followers, as many people are only following your account in the hope that you'll follow them back so they can broadcast to you. It can feel like everyone has a megaphone and no one is listening.
That being said, it is possible to reach your target audience via Twitter and engage with them for mutual benefit. I suggest using HootSuite to find your audience by the subjects they are likely to be discussing and politely adding to the conversation and following those people.
Place your emphasis on quality, not quantity. There are Twitter users who have upwards of 100,000 followers, but they are all talk, all the time and people are most likely ignoring them. I'd bet there are other Twitter users with 100 followers who are actually getting more results from their efforts because they placed their focus on quality.