Email - it's open (because spammers can get your "name/address" and send tons of offers to your account.
Social - right now, we have an authoritation (fb/twitter) that filters on a high-level (instead of personal spam filters or pre-formulated block algorithms).
But if social was more like email (open), a spammer can generate a false identity - aka (semore b., ryan private, john smith, john swiss, john jones, etc). Then we would have to do the same thing that we do now, explicitly forced to indicate non-correspondence.
Other abstract/theoretical extreme cases are
1) laissez-faire vs socialist economy
2) evolution vs intelligent design/evolution