If I may start from your good points: 1.) In a network of 500 million users already are the scaling problems still open? I would suspect that the biggest scaling problems appeared much earlier. If somebody knows what are the scaling problems that such an enormous networks faces would be very interesting.

2.) I would guess that top programmers can have top benefits in several companies. Even less excellent programmers could get a decent paycheck in many places

3.) I am not familiar with the demographics of facebook but my gut feeling is that most of the programmers are in their early twenties or mid-twenties. This was also my impression during one of the job fairs that I have attended.