I'm one of the people who friend everyone back to be polite, In the early days I tried to read everyone on my flist, but it quickly became impossible.

I tend to read and comment most in the journals of people who are A) RL friends or people I've known across several internet fandoms B) people who are still actively posting about BtVS fandom, and C) people who comment in my journal. But a lot of the time, even when I'm only reading a small portion of my flist, I'm skimming desperately, trying to keep up, and there's no way I can answer every post I read.