The site doesn't even seem close to being useful or complete yet. I think it's intentional.

They didn't want search traffic until the content and structure start to stabilize? Google, et al. will crawl for years after a URL 404s so it might make sense to hold off until the first round of stabilization

Maybe someone from Facebook made the robots.txt file, ayyy!

The site was in super-alpha. I think it was intentional.