Hacker News Reader

Is this by Facebook? The site implies it is, but it's not clear.

Yes, it says "Facebook Open Source" at the bottom of the site.

True, but that page lists Facebook's projects, but not Nuclide. Nuclide is also not on the Facebook Github.

I think it's because they didn't launch it yet, probably it's a private repo.

Yes, note the "Facebook Open Source" link at the bottom of the page.