[anujkk]

It all depends on what kind of community/website it is.

Communities like HN/StackOverflow - Why do people contribute here? Though there is reward system is in place here(karma) but I doubt if that acts as primary motivation. In such community, my contribution is your reward and your contribution is my reward. YC isn't getting any direct monitory benefits from HN but having a good successful community of hackers helps them indirectly in many ways. So, it's a win-win situation for everyone.

Products like Google, Gmail,Facebook etc. - Isn't it rewarding enough in itself to be able to use such excellent products free of cost(even if it comes with minimal text ads)?

The bottom line is that if you are using a website because it is providing some kind of value to you and your contribution is actually helping it in providing even more value back to users, you shouldn't think about monitory awards.

If it is not providing anything valuable, you won't be using it, even if it is free.

If you are getting paid for using something or contributing, it means the site in itself isn't that valuable to you. So, they lure you through monitory benefits. For example, paid surveys.