Reason why we post is as mentioned, a form of socialization with others.
You know Arcanine... you don't have to talk ever. THere are mutes who are living perfectly well... but i wonder why they want to speak again? Without words they have difficulty communicating with others. Communications make every day life different from every other day. Posting is a form of communication. It's about the ideas and things within a post that makes posting fun, not the +1 you get form it. Interaction with people means communication. Through communication ideas exchange from one another, opening up your thoughts and mind.
Posting is exactly the same as why we talk, why we play (even if it's "one player" video games... it's interaction between you and the game producers who put their thoughts into the game), or why we do almost everything. Posting should be something everyone does, but just that the point of posting is lost now as people shifts their attention from the interaction to the +1. But in the end (even if you're just seeking for +1), everyone post because of interaction with people one way or another.
SPAMmers spam because they want to be noticed in one way or another, therefore it's still interacting with somebody, sending his/her ideas to another person. I can promise you that no SPAMmers will invade a board that has no one in it. They also seek interaction and response from someone else (which is ultimate, interaction of some form.)
People who want those +1s post because they want a higher number. The higher number can and does give the person a sense of superiority, or higher status. People with higher post counts will get admired by others. This is also an interaction between people.
And then finally... some people post because they want to speak their mind and thoughts regarding a topic, chatting with people to have a fun time and to share their opinions. This is obviously, another form of interaction.
So in the end... posting's ultimate goal is communication and interaction between people. Just that everyone demands different forms of interaction and communication... some want attention, some want power, and some just wants a good friend to talk to. Everyone post, because everyone interacts with one another.
(sorry for the poor organization in terms of structure within this post... not really well thought out =/ )