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?
This probably used to be true, as you can see from this Forbes article. It shows that somebody held almost 440.000 bitcoins in a single bitcoin address in the summer of 2012, hence this person's bitcoin holdings were at least 5 % of all bitcoins in existence (there were about 9 million bitcoins in existence at the time the article was written). Note that these bitcoins have since left that bitcoin address, but that doesn't mean they disappeared or that there are no other bitcoin-rich accounts. And who knows, maybe people with that many bitcoins split their wealth across more than one bitcoin address...
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.