Here is a paper (PDF) by Bonnie Nardi, Diane Schiano, Michelle Gumbrecht and Luke Swartz about motivations for weblogging:

We discovered five major motivations for blogging: documenting the author’s life, providing commentary and opinions, expressing deeply felt emotions, working out ideas through writing, and forming and maintaining communities or forums. These motivations are by no means mutually exclusive, and can come into play simultaneously.

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