Free and open-source software/Free riders
So what about free riders? What happens when there are people who only take and never give anything back?
Well, nothing really. Contributors are not expecting a specific quid pro quo arrangement — they get plenty in return from using other things in the universal commons. This is the essence of a post-scarcity economy; when a resource is abundant, it can be given away without expecting anything in return. If it is trivial to duplicate the results of someone's efforts then the more people who are able to make use of it, the better. The situation is not zero-sum — people are not going without as a direct result of someone else having it.
A situation of abundance currently exists with regard to information on the Internet; it is free to replicate, I can give it to others without going without it myself, and as a result, people give it away freely and take it freely.
Consider free and open-source software where anyone with a computer and internet connection can download the Firefox web browser or OpenOffice office software for free. Most people will be consumers rather than contributors, but this is of no consequence.
After a while of enjoying the fruits of open-source, many people are only too happy to contribute in some way, giving back to the community that has provided for them. The fact that they are not required to do this, in many instances makes it more likely that people will do so, uncoerced. This is human nature.