I guess technically it’s not people you “don’t know” but more like “barely know.” Per my OP, I understand people using Facebook the way you say you do - basically an online contact list you use to keep in touch with old friends or whoever, and you rarely or never post on your wall or read other people’s walls. That makes sense; contact lists are useful.
What doesn’t make sense to me are people who seemingly use Facebook like you describe, but who also go ahead and friend people they barely know as well. That’s what has me going 
when they friend me. Why friend me, if not to get to know me? And if you’re trying to get to know me, friending me but never ever ever posting things on your own Facebook page isn’t advancing a friendship - it’s too one-sided and tweaks my creepy meter.
Maybe I’m just ultra-popular and people who barely know me gain status just by having me on their friend list.