Do not contribute free work to SO, Wikipedia, or any other site that seeks to replace the web with its own brand. Instead, if you have useful knowledge to share, write it up on your own website. People will find your information using search engines. And when these agglomeration sites dry up, search engines will regain some of their lost usefulness. If you get good at writing you can even get paid for your articles, and the editorial process will make them even more useful for readers.
SO, Wikipedia, and the like not only take and use your work for their own purposes, but are bad for the web. Their effect is to move the web away from a distributed network of knowledge under the control of individuals responsible for their words, and toward homogenized, voiceless near-monopolies of information. They are nearly as unhealthy for the web as Twitter and Facebook.