They were there before 1.06. After 1.06 the leaderboards have been whiped clean because of the change in physics. New times set after 1.06 till 1.07 are also gone if i'm right. It really sucks man. All my times are gone, but still got the gold. I used to set times against my friends and grinded so hard for some of times on the leaderboard that they are almost impossible to beat. So maybe i never will get a time on the leadersboards on a single campain mission. PD being PD...
Except that you can't now get a time on the board unless you beat your own previous best, which can be a real challenge if you put in the time to beat your friends originally. The tweaked physics helps get faster times, but it's still annoying.