[presty]

Not to mention that he had built several other things prior to starting Facebook.

There's also a lot of libs/services he can use

And if he needs help in understanding or implementing something, he can always ask for some help from someone @ FB...

[ericjang]

This. I think most of the work that goes into building a successful large-scale project is making the design choices that minimize required effort.

Everything after that is a matter of typing correctly and adjusting design choices to unexpected constraints.

Having the knowledge of FB's engineering at his disposal will certainly make things easier.