They're explicit about this:

"Pants models code modules (known as “targets”) and their dependencies in BUILD files—in a manner similar to Google’s internal build system" [1]

But I have to wonder why they choose to spell out "dependencies" :-)

[1] http://pantsbuild.github.io/announce_201409.html

> I thought Facebook would open source theirs first.

Well there is Buck, Facebook's Java and Android build tool: https://facebook.github.io/buck/

It's not really a generic tool like Pants is, but it's something. Thanks for sharing!