What is the purpose of blogging?

A discussion with an online friend lead me to question what the end purpose of blogging is.
Why do people blog? Why should I?
My friend tells me I should blog for myself; to entertain myself first. I’ll be mulling that over, but my initial reaction to that is that’s akin to writing books because I enjoy writing books for myself knowing that they will never be published and never be read. What’s the point in that?
Blogging has been described by some as a diary or journal open for all the world to see. Isn’t that an oxymoron? Publishing private thoughts?
No, my first reaction is that, by definition, blogging is an open, public and mostly egotistical form of self-expression. I blog because I enjoy it when other people read my posts and say things like, “I really enjoyed reading that,” or, “I’m glad someone else thinks the same way I do,” or, “I’d never really thought about it that way until reading what you wrote.” In nearly two years time, I’ve heard very little of that. I’ve heard plenty about mortgages and naked teenage girls and Viagra, but precious few in the way of comments.
Please understand, I’m not asking for a slew of pity “Oh, I really do enjoy reading your posts” comments, I’m just trying to decide whether spending my precious little free time sending messages in a bottle out to God only knows where is ultimately worth it. I don’t want to be just another mediocre blogger.
Maybe I’m just tired. Maybe I’m starting one of my infamous melancholy swings. I don’t know. What I do know is that I’ve written, by most people’s standards, some pretty good stuff lately and I had to beg for comments! That tells me no one cares what I think and, if that’s the case, why continue to blog?
Maybe I need to rethink how I’ve structured the blog itself. Maybe I’ll turn it into a bulletin board or just aggregate other posts I find interesting or begin collaborating with others. Or, maybe I just need to stick a fork in it.
Maybe I just need to mull it over for a while.