Some of those boundaries are matters of personal preference. For example, I try not to attack people who aren't public personalities. The careful reader will know that for at least a few years, that boundary was blurred on matters relating to my ex. I've made occasional unkind references to my fellow New Jersey drivers, though never by name. I rarely reference movie stars, because I don't really care about them. Book characters are far more real to me and sometimes I've been critical of them or the author who created them. And I pull no punches when it comes to elected officials, whose statements and actions are fair game for criticism because they are public officials.
Other boundaries are matters of decency. I have posted pictures of people in my life, but never without the confidence that it's okay with them. Children are never identified by anything other than their initials, as are most people I write about. I do write about my job, and the sentiments I express in those posts are genuine and true. But I am typically vague about specific school identity issues and I never post unkind comments about either the kids I teach or my colleagues at work. If I have any doubts about how a post related to my job will be received, I err on the side of caution and I don't post.
I regard my Twitter account as an extension of blogging me and mostly use it as a device for making smart-mouth comments. There are some useful professionally-oriented Twitter communities out there and I sometimes follow their exchanges. However, I almost never add my comment to those "conversations" because I don't typically consider Twitter a professional place for the real me, just a place where the digital me hangs out to be amused.
I post as myself on some of the political blogs that I read because I consider them to be professionally-oriented and I am a trained political scientist who is willing to own her ideas. At blogs that I read for personal reasons, I post as Sassafras Mama. I don't see anything contradictory or even confusing about this policy. I see the lines as rather starkly drawn and I almost never have confusion about who I should be when I am in the digital world.
Even so, I've thought about these matters a great deal of late, because it seems like there is increasing pressure to merge our personal digital identify with our public digital identity. I find the pressure distinctly uncomfortable because of its implication that a private identity is no longer an option in a 24/7 digital world. I don't agree and not because I have any nefarious goals. If a current or former student read my blog, it wouldn't matter to me. My boss is welcome to wander over and read what I write. I don't go out of my way to avoid such events, but neither do I encourage it. I think there is room for a boundary between the digitally professional and the digitally personal. I want one in my life. But I sometimes wonder if I'm the only person who feels this way.