http://www.gao.gov/products/gao-13-516?utm_source=twitter&utm_medium=social&utm_campaign=gpi
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Virtual economies and currencies pose various tax compliance risks, but the extent of actual tax noncompliance is unknown. Some identified risks include taxpayers not being aware that income earned through virtual economies or currencies is taxable or not knowing how to calculate such income. Because of the limited reliable data available on their size, it is difficult to determine how significant virtual economy and currency markets may be or how much tax revenue is at risk through their usage. Some experts with whom we spoke indicated a potential for growth in the use of virtual currencies.
Link to pdf http://www.gao.gov/assets/660/654620.pdf