Probably a better way to figure out the real employment numbers behind eCommerce is to trace the economy behind an average persons yearly consumption with and without Ecommerce. If the average person is spending more with eCommerce then you're going to have more employment due to the bigger economy behind it. (Assume $1000 spent by the average person in both cases result in equal employment.)

You can then trace second order effects by comparing where the employment is occurring and in what industries.