The third sector grew out of the need to counterbalance the extraordinary influence of the private sector on economy, society and environment.
Which is why it's not surprising that:
a. Most NGO models don't work very effectively (see 1970s tactics: write to your congressman? sign a petition?)
b. When NGOs try to harness the ways of doing business that, well, most businesses use, but with a goal of affecting positive change, they often get into awkward territory
Maybe the best way to rethink sectors is the reality that everyone is an activist now.