youronlyone, This is the current plan and stance of #Threads / #Instagram regarding enabling #ActivityPub / #Fediverse for their users:
Personal analysis:
Since most users wouldn't understand what ActivityPub / Fediverse is, it is likely there will be far less than 40% or 30% (guesstimate) of their userbase who will appear in the Fediverse network.
It will highly depend on how they will explain the benefits of enabling ActivityPub in their accounts.