Depending on how philosophical you want to get, there'll be people who derive pleasure and purpose from sharing what they know or building something that is enjoyed by others. There'll be people who gain pleasure from being known for being helpful/useful/knowledgeable or even being recognized at all. Even if we can't call it altruism that's not necessarily a bad thing. I guess I just feel that there are, and likely will always be, plenty of incentives for people to create and share online besides financial ones.