Given the market cap of Facebook is $245 billion and it has 1.44 billion active users, then each user is $170 (170 == 245/1.44). If each user generates $1 for Facebook each quarter, that's $4/yr which is an annual return of 2.3% (2.3 == 4/170). I'm not sure where I stand on calling users "sharecroppers". Seems a little unreasonable. But the economics of it are interesting and concerning.
They're generating closer to $10 per year for Facebook. Your annual sales take jumps to 5.9%. A year from now that will probably be closer to 7%. PE ratios usually compress with time due to slowing growth. I would be comfortable predicting that Facebook will eventually get this calculation up to 15%+. Their PE ratio right now is 80, which is producing a massively warped calculation on the return %, as that PE compresses heavily over the next ten years, their return per user will skyrocket.
If their PE ratio were a more sane 40 right now, this number would already be at ~12%. Double their annual income to $6 billion and their sales to $30 billion over a few years, drop their PE to 35 ($210b market cap), boost their user base to 2 billion, and it jumps to 14%. A very plausible outcome four years out. This is all calculated off of sales of course, not income.