Not to mention being a top contributor to the Linux kernel, clang/llvm, mysql, doing almost everyone's security research for them (including, notoriously, finding most of the issues that get released in Apple security patches), etc. Facebook sure does a better job of evangelizing their own open source stuff, but I'm not sure that means they're actually contributing more. Even some of Facebook's "contributions" are ripped off from Google, like buck.

Buck was not "ripped off" from Google any more than Hadoop was "ripped off" from Google.

Buck is a nearly verbatim clone of the build system used inside Google, except that the word "blaze" has been replaced by the word "buck", and it was written by xooglers at facebook.