Its easy to create accounts later.
You can allow accounts which have kind of history only. AFAIK Facebook accounts have a number. Maybe its ascending.
but just looking at the history roll of the profile should be enough. If there are entries older than XXX then its fine.
You're right, there are many fake accounts. No method is perfect.
The alternative I had in mind was 'gathering DNA probes', but then you have a point of failure from real world to submitting it.
Its huge amount of effort, doesn't scale easily .. is expensive etc.
You cannot be fair or to be more accurate: There are many ways to define fairness.