I suppose that people do what they want to do, but it's so transparent when someone makes a post that is clearly intended to be read by someone, perhaps because they don't have the wrinklies to say what they mean directly to the person's face.
That's one thing the Internet has been generally good for in some cases though, at least enhancing communication. I look at how high school kids now have a couple more "safe" avenues to communicate (e-mail, IM), and it's probably a good thing. The only negative is that some kids are even bigger social retards when it comes to real life human interaction.
Me, I tend to just write whatever's on my mind. I admit that I've written stuff that might be read by people I'm talking about, but generally it's not anything I didn't already tell them directly.