Holding of 9% of all bitcoins? In this discussion:
https://bitcoin.stackexchange.com/a/9057/10490
One of the users mentions a wallet with 9% of all bitcoins.

My question is: Is this true? What could be the source of this information? Is the identity of users not kept secret?
The blockchain is a public ledger and all transactions can be viewed.  But it doesn't contain any names or other personal information.  So you could determine that a particular bitcoin address owns 9% of all coins, but you couldn't determine who has the private key associated with that address.

That said, you can follow a chain of transactions and make inferences based on the addresses used.  For example, if address A and address B are both used as inputs in the same transaction, then you can assume that the addresses are owned by the same individual.  If you sent coins to somebody at address A and you know their identity, then there is a good chance that you know the identity of the owner of address B as well.