Like I said, it isn't rational, but I tend to get kind of nervous about being too easy to "find" (which makes no sense since I have a personal webpage with my professional contact information, etc.) so the idea that folks can see when I'm on and what I'm reading at any given time kind of weirds me out.
Back when I first made a webpage (in college) it was during a period where it was unwise, if you were female, to even put a picture of yourself online lest you receive a zillion emails from freaks who would write to you simply because you had two X chromosomes. I realize that the Internet is no longer the sort of frightening, unknown place that it was back then, but remnants of my prior caution still remain.
I've also been stalked in my time, which makes me a bit more circumspect. Not very logical -- if I were truly worried about it I wouldn't have a Web presence at all and I certainly would not spend as much time as I do participating in online bulletin boards. But like I said, old habits die hard.