In mybitcoin.com 's terms of service agreement, it is stated that a historical transaction record of an account is kept.  This information combined with the Bitcoin block chain, may provide a complete transaction path of a Bitcoin passing through mybitcoin.com - maybe even if you do not even spend your own Bitcoin from mybitcoin.com (that was originally sent to mybitcoin.com).

See:  mybitcoin.com Terms of Service Agreement when signing up:  

"9.2 Unless ordered by a Court of competent jurisdiction, MYBITCOIN LLC shall not reveal User's contact or identifying information or transaction history to any third party."

It becomes clear that whenever something is centralized, anonymity is lost.