Kaye Trammell has a excellent podcast about blog research that I recommend that you listen to. Some of her comments in regards to what motivates people to blog are:
– Bloggers are mostly motivated by self expression – a will to share thoughts and feelings with others.
– People who give feedback in form of comments are motivated by social interaction – a desire to interact.
– People who give feedback in the form of trackbacks are motivated by information – a will to share interesting information.
I believe this graph is associated to this research.
Dr. Trammell also mentioned a Polish blog survey that found that bloggers in Poland are more often female than male and that female bloggers are more motivated to blog by social interaction reasons than male bloggers. Polish blogs are often password protected so they are not open for everyone to read. It is also common in Poland that people steal others blogs, which means that they try to find out their login and password and then take over the blog.