I've been an occasional commenter on others' blogs for at least 3 years or so, and until very recently, I have usually insisted on using "Mr." or "Ms." whenever I knew the last name. I have, in the last month or two, started using first names almost all the time. It's much less supercilious to use first names, and when I use last names, I think I usually come off as cloying and saccharine. It also takes longer to write "Mr./Ms. so-and-so," especially because a large number of last names are either longer or the same length as the first names, and so adding the "Mr." or "Ms." simply requires more writing than I want to do.
I think I used the last names because I thought it was a way of engaging people politely and respectfully. I still use last names with people I don't know. And I use them with some professors: as a general rule, I like to call my professors "Mr." or "Ms." instead of "Dr." because as far as I'm concerned someone with a PHD in history is no doctor. (Doctors are people who take care of your health and prescribe medication. By that standard, I call my nurse practitioner "Dr." and I would call her that even if she didn't have a PHD in public health in addition to her nursing degree.) I still relent when I talk to a professor who seems like he (usually it's a he who this applies to) might be exceptionally sensitive to being called anything other than "Dr." Of course, there are a lot of professors I call by their first names, and most of these are those who are so known by their first name in the department that it seems ridiculous to call them "Mr." or "Ms."
I think I got my preference for using last names when I was a bank teller. We were encouraged to call each of our customers by name. I don't know if we were also encouraged to call them by their first name, too, but that was the standard practice. Most of my customers were older than I (I was about 23 or 24 at the time), and I found it ridiculous to call people by their first names when I knew them only because I cashed their checks or accepted their deposits. So that's when I developed a policy of calling people "Mr." or "Ms."
Anyway, it doesn't seem to work with blogging. So I've been going back to first names.