I've noticed on my iOS Facebook apps that, under the chat feature, they display how recently it's been since my friends have been active on Facebook. In other words, the last time they accessed their Facebook account (minutes, hours, etc). This could easily lead to some animosity if you send a message to someone and later saw that your friend had accessed Facebook but didn't read or acknowledge your message. You might think, "Hey! Why is so-and-so ignoring me?"
It took a lot of searching to figure out how to turn off this time tracker (which, in this day and age, means more than half an hour on Google followed by some experimentation). It turns out there isn't a specific setting on either Facebook's website or in the iOS apps to turn this off. The only solution is to completely log out of your Facebook apps on your iOS devices. Since this isn't very practical it looks like iOS users are stuck with this creepy feature that broadcasts how long it's been since they last looked at their Facebook account.
Fortunately, this time tracking feature doesn't seem to be an issue when on the desktop or Blackberry. I wonder what the Android user experience is.