Why do people contribute content to the Internet?

There are several reasons to contribute content to the Internet.

I will mention some:

1. There are advertisements in the site and due to visitors in your site you are giving exposure to those adds, thus you get paid.

2. You wand recognition of your work, your knowledge or just yourself, so you post interesting content or something useful and make yourself a name.

3. You have a community you need to provide with knowledge (Students, Employees etc.) so mostly the easiest way is post on the net.

4. You are a nice guy...

The internet is a really huge place, and a lot of people are doing a lot of things for many reason, I can only speak for myself: For me it's for the same reason that I make art or talk to people - to create and interaction and to express myself as an individual person. I think that one of the things in common to almost anybody in the world is a will to be heard, to be listened to or seen. I think that a lot of people express their inner thoughts or feeling in different frames - may it be in a therapist office talking about feelings, or in a political debate at lunch break or by writing a diary or spray painting their neighborhood. What I like about contributing to the internet is that it exposes my writing and art, and mostly, myself, to a variety of people who would have never seen me or it and also that the boundries between a viewer, an artist and a participant are not as clear as in the offline reality. I really like the way the the internet incourage action (n commenting or responding) in a large social group. while other communication tools are either very personal (phone call, conversation with a friend, diary) or very passive for the user (like TV, radio or newspaper.

I believe the main reason is the fact that people wish to show knowledge for gratitude. It's always nice to share knowledge or past experiences and then be flattered on'em. 

Well, first, it's important to note that actually not many people contribute content on the internet. In fact, research shows that in many services most of the content is contributed by a small percentage of the users. 

Check out Jakob Nielsen's Participation Inequality: Encouraging More Users to Contribute post, which provides more data on this 90-9-1 % user distribution behavior. 

Two more additional interesting readings on this fascinating topic:

Why Do People Contribute to Digg or Wikipedia? by Diane Ensey

Why do people contribute? (who is dirtyfratboy?) by Jason Calacanis

I ran into this

"Surprisingly, those who were rewarded for doing so were less inclined to continue. "The underlying explanation for psychologists is that when you pay somebody to do something ... they get the feeling that they're not doing it because they enjoy it," Barber said."