Why do users participate?

I've been thinking about why users participate on websites and services. I have this gut feeling that too many 2.0 sites exist on borrowed time.

Having recently re-read Darwin's Origin of Species it occurred to me that participation or altruism must be hard wired into our genetic code as there are examples of it throughout the animal kingdom. For example, recently it has been proven that cleaner fish have reputations. Dr Lexa Grutter, from The University of Queensland's School of Integrative Biology, along with colleague Dr Redouan Bshary, from the Swiss University of Neuchatel's Department of Zoology, have shown client fish eavesdrop to determine the trustworthiness of cleaner fish and cleaner fish in turn behave altruistically to be considered more trustworthy.

Those species who understand participating in something for the greater good of the group and ultimately themselves are ultimately fitter and more likely to survive.

There are four Darwinian reasons why people are generous, altruistic, moral towards each other via websites. In no particular order:

1. Genetic kinship: genetically predisposed to ensure the survival of our genes via our offspring.
2. Reciprocation: works best in closed or small groups where you know the favour will be returned.  Again, interactions between the cleaner fish  and its client fishes are a well-known example of mutually beneficial behaviour involving different species.
3. Acquiring a reputation: increasing your standing in society through reputation. 
4. Buying advertising: Individuals and some species increase mating success through costly displays of altruism. Ostentatious generosity and public risk taking.

In the future, sites that fails to the needs and reasons for users' to participate will see a decline in usage. In the very least participation will appeal more and more to very new or very loyal / traditionally engaged audiences as opposed to mainstream site traffic.