not sure but I think its part of the invisibility option. when you set your profile so that others can not see when you are logged on that also makes it impossible for others to see when you were last on. so when you look at other members profiles and do not see when they were last on you know that they have their "invisibility cloak" on. sorry couldnt resist the harry potter reference.

that doesnt mean you cant tell if they have been on. if they have posted while they are in invisible mode their post still shows up, so you can still look at their statistics to see what they have posted and when. you can also still pm your friends when they are in invisible mode. so if there is someone you are looking for maybe drop them a pm and if they are online and want to talk they can pm you back.