Etherscan displays all accounts in ethereum being used in the following section https://etherscan.io/accounts
Question: How can I get a list of all active ethereum accounts using geth?
Possible answer: Use Parity. It enables this feature by the RPC call to parity_listaccounts.
Why Not an answer: The account information is available at https://rinkeby.etherscan.io/accounts and rinkeby runs only geth. So geth must be able to provide this information.
A similar question: There exists a similar question How to list ALL Ethereum addresses with a positive balance, but that requires traversing each block which is very time costly process.