The problem you are addressing is interesting, at least for me.
What about this pattern:
a) the user has a switch that has two settings (s1, s2)
Setting (s1) allows detailed informations, like 'one hour ago', 'is on line', etc.
Setting (s2) allows only statistical informations to be known, like:
intensity of their presence online (average)
has not been active since 5 days, one week, etc.
On the programming side the problems are not relevant.
I like to see this site, as a big public library, when you enter a public library and
move around, you will recognize and (silently) greet your friends, when you see them
sitting around a table. No reason to hide in a public library, no reason to hide here,
but you may be free to do that, for instance if you don't want a particular person
to bother you.
Alex, I agree with Miriam.