one of the complaints I see from Facebook users is that this feature brings their accounts to the attention of exes, stalkers, or just generally people they dislike who (as is usual in such situations) share the same social circles. I wonder if in order to avoid that we would have username-identifiable analytics disabled by the endpoint user by default, with an option to turn it on.

It could end up being something like a couple of the LiveJournal features, where if you opt in to see what other people are doing, you are by default opting in to being seen?

I don't know how feasible this would be from a user perspective; I wouldn't use it.