According to professors Peter Kollock and Marc Smith motivations for contributing to online communities do not rely on altruistic behavior on the part of the contributor they depend on these four pillars (Wikipedia articles: Virtual community, Egoboo and Sense of community).
Anticipated Reciprocity. A person is motivated to contribute valuable information to the group in the expectation that one will receive useful help and information in return.
Increased Recognition. Recognition is important to online contributors. Individuals generally want recognition for their contributions.
Sense of efficacy. Individuals may contribute valuable information because the act results in a sense that they have had some effect on this environment.
Communion. People are fairly social beings and it motivates many people to receive direct responses to their contributions.