Yes, a person can contribute to the social sector while employed in the corporate world, because corporations often do things that help people. A company that makes medical equipment is contributing to the social world. A person who is an attorney who offers charity work is contributing to the corporate world, even though they work privately.
Contributions to the social sector can vary widely from charitable donations to volunteering someone's time. People work in the private sector and contribute to the public good every day. Just look at corporations who sponsor charitable runs, walks and other events to raise money for various causes. There are as many ways you can contribute to the social sector as there are humans on planet Earth.