Nearly 30 million monthly uniques according to Quantcast - placing IRS.gov as the 26th biggest website. The 30m uniques represents 10x growth since January 1st:
Those are very big numbers. But more interesting (and obvious!), if you extend the above chart to twelve months, you have the perfect (and most exaggerated) definition of seasonality. Not so obvious: IRS.gov continued to grow through April and peaked in June.