As for Facebook's revenue-per-user based on revenue-per-employee and number of users, you'll need the number of employees (not just engineers). That's about 2000, which works out to $4 per user. This is exactly inline with what was documented in this nifty chart: http://wallstcheatsheet.com/wp-content/uploads/2011/01/chart...
Amazon is kicking ass. Apple only makes $150 revenue per iOS user (Asymco).
Why should we be surprised that the company whose business is in shipping and selling a wide range of physical products has more revenue than a range of companies whose income are either service fees or advertisements? We're not comparing like with like; Amazon's revenue is counted against by buying and stocking the physical products they sell to derive that revenue. None of the others on that chart have physical product to worry about and so retain a much higher percentage of revenue for themselves.
Oh, seems that Facebook makes 1.2 dollars per user from your numbers