Facebook playbook for market rise is legendary. The limited rollout, the college based communities based on your edu email created this campus privacy and campus group. Starting off with the ivy league schools and slowly working into other schools created this demand as people talked. By the time facebook opened to the general public they had such a buzz. When they rolled out to this group they included one killer feature.. they allowed you to give your hotmail email/password and they would get a list of your contacts from your email and invite them to facebook. That brought in your aunt, brother, old friends to facebook. That created a network effect. Throw in the whatapps story and instagram story and an election/congressional hearings and you have facebook today.
The code part seems so minor. Retracing their steps is impossible. The path to facebook killer is a huge challenge to think that could only be done in a weekend is crazy.
I think it's really easy for tech teams to do things in a sub-optimal way and then get all caught up in fixing problems of their own making and start to think they're doing really great technical work and that it is a competitive advantage for the company. More companies need to face the fact that their software can be easily replicated and that the value lies elsewhere, such as brand, reputation, reliability, good customer service, etc -- other things that an intern can't replicate in a weekend.