@evan to put this another way: we either have a notion of somebody's "space" as opposed to just their account, or we don't - and currently we don't just as xitter doesn't
if we don't have "space" to post to, what I'm suggesting is the most privacy-preserving option
i'd be entirely cool with adding a notion of spaces and everybody having one of their own by default ("communities" being another example that's not always owned by exactly one account), except if we've got that far i want the option of "exactly these people" filters too