I can't begin to fathom a motivation for that. Can anyone think of a reason, or is this likely simply a mistake?
Maybe someone from Facebook made the robots.txt file, ayyy!
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
The site was in super-alpha. I think it was intentional.
This has already been changed to
#User-agent: *
#Disallow: /