A question

I’ve been thinking a lot about blogs and online journals recently.  To be honest, not in any consideration of what I’m doing with my own so much as just being very interested in them as a form or mode.  I’m interested in knowing what other people think about them.  What are they for?  What are some of the various reasons why people keep them?  How do they decide on what to write?  What sort of content do they choose to make public versus the old idea of the private journal or diary and why?  I’m interested because I think the blog or online journal has its origins in the form of the journal or diary or log, and those are (mostly) private spaces, but an online journal is in a public space, and I think this makes it something very different from the private mode where its form, its structure at least, originates.  I know some people use them as public platforms for their business or career or their politics, which falls in line with public use of a public form, but what about the journals that seem more oriented to private matters being posted in a public space?  Does it indicate that certain things we once considered private matters are transitioning into public or social matters?  All sorts of questions I have about this stuff, but I don’t have a sense of what lots of other people think, to be honest, so school me.