How does anyone know an address was a MyBitcoin address?
How does anyone know an alleged address is really not a MyBitcoin address? What if someone just scrapes addresses from the block chain and claims they are MyBitcoin? Seems like quite a stretch to me.
I was about the ask the same question.

1. We took addresses from posts that were posted prior to our announcement. These people had no reason to fake their MyBitcoin address.
2. The large amount of money transferred through and still residing in these addresses confirms, to some degree, the identity of the wallet.
3. The wallet contains many publicly-posted addresses, posted by different people.