Actually, only you can see that. I read about that once somewhere on here. And I just confirmed it, because I just looked at your profile, and it says nothing about what you're doing now, or the last time you were active. (Though I do know you're online, because we're contacts, but that's it.)

EDIT: That is to say (for anyone wondering), if you put yourself in invisible mode, you will still see on your own profile your current activity, and there's no indication that others CAN'T see it, but I'm 99.9% sure that they cannot.