Note that this may be a better model of friendship, but according to TFA we ought to be looking for a measure which is symmetric: if we ask X if Y is their frie

Note that this may be a better model of friendship, but according to TFA we ought to be looking for a measure which is symmetric: if we ask X if Y is their friend, we'd like to get the same answer as if we ask Y about X. With that desideratum in mind, any measure proportional to "[non-work-related] hours spent voluntarily together" is relatively symmetric.