Panos Ipeirotis provides a valuable resource in mTurk Tracker. This website not only tracks information on the HITs posted to Mechanical Turk, but also the participants found on the site who are willing to participate in surveys. Currently, it demonstrates that the workers are balanced in gender, typically 26 years old or older, balanced between being in a committed relationship or single, and mostly earning less than $40,000 USD per year. As to country of residence, the graphs show around 70% of workers are American, 10% are Indian, and the remaining 20% come from all other countries.
This ILO report on the on-demand economy also explores the demographics of CrowdFlower and Amazon Mechanical Turk workers and offers great data on the demographics of both platforms.