It may be the case that Amazon Mechanical Turk WorkerIDs are not anonymous. Lease et al., 2013 describe at length how personally identifying information may be exposed when a researcher shares WorkerIDs. It is unclear to me the extent to which Amazon constructs their WorkerIDs at present, given that one of their striking demonstrations did not apply to my WorkerID. That is, they describe simply googling the WorkerID and receiving a picture of the participant, along with their full name.