Dave Winer:

A good blog exists independently of people reading it. Even if no one read my blog, I'd still write it.

Matt Mullenweg:

Blog just for two people. First, write for yourself, both your present self whose thinking will be clarified by distilling an idea through writing and editing, and your future self who will be able to look back on these words and be reminded of the context in which they were written. Second, write for a single person who you have in mind as the perfect person to read what you write, almost like a letter, even if they never will, or a person who you're sure will read it because of a connection you have to them.