If Facebook would begin again what should it use PHP or JAVA (J2EE)? Why?

Special factors:

  1. Scaling
  2. Development speed
  3. Maintainability
  4. Development tools
  5. Development costs
  6. third party programs
  7. Security
  8. Deployment speed

PHP clearly.

The skillset is 100x more prevalent, it scales another way, and you will iterate more rapidly. Finally, the abilities &lifier practices needed to scale it are very well-understood and established. Toss in the truth that they are able to hire core PHP designers - I understand they've 3+ within the company now - also it makes much more sense.