[netsharc]

I would think having a week or a month's worth of data is enough to get rid of such people who are "background noise", and get an accurate enough "who's talking to who" mapping. (Something like, "A and B's presence are within a minute of each other for 80% of the time, even during 'off-peak' times, for example early in the morning.").

We are worried about the NSA collecting metadata, but you dismiss this as an end user problem. Famously this is why Facebook has its settings to be opt-out instead of opt-in, because a high percentage of users never changes the default.