The thing is, the nature and terminology of LJ/IJ often can give off the impression that we know the people on our flists. Hell, they're called *friends* lists, that right there is misleading, we call these our "journals" which implies that we're writing personal stuff here. In some cases people *do* write personal stuff, in other cases people don't. I know that it can be very hit or miss with me, where one day it seems like I'm bearing my soul for all to see and other days I'm making meaningless posts about the hotness factor of my favourite shows.
I think the thing to remember though, is that just because you know someone online, does *not* mean you know them. You know one side of them, but you can't know all there is to know about them. Even if that person is someone who seems to pour their hearts out on their journal. I know that there have been periods in my life where I've done a lot of soul searching on my journal (early 2005, especially), but I think there's something everyone should understand, people act differently online than they do in RL.
I know that if I make a post about how I'm having a fight with someone else, most of you who comment will be on my side. I know that if I talk about having a shitty day, others will come out of the woodwork and commiserate. This makes me feel more comfortable doing and saying things I would never admit to in RL. Case in point, I don't complain when I'm sick. I grew up in a household of hypochondriacs so I try not to whine when I'm ill; not that anyone would have ever guessed that about me based on my entries from last week where pretty much all I did was whine and complain and generally be a big baby about being sick.
I guess what I'm trying to say is that the person you know online might not be anything like the person on the other side of the computer screen. Someone who is constantly ranting and complaining on their journal may be doing that because they never do it in RL. They may come crying to their flists when things are tough because they know they have their flists full support. They may speak in bawdy terms and crack jokes every other sentence but are incredibly shy offline.
So if you think that just because you read someone's blog you know them and have any sort of right to comment and/or criticize on their RL, their choices, their marriage or long term plans for their life, well, then you're wrong. Because you really don't know enough about the person you're commenting to to make those statements, you know a slice of them, perhaps an overly honest slice, but still, just a small part of the overall person.