Right. None of which you could reasonably describe as "the most important pieces of its back-end infrastructure," unlike HipHop, Thrift, HBase, Cassandra, memcache, etc. at Facebook. Google has contributed a ton of wonderful products to open source; however they will never contribute BigTable, MapReduce, GFS, Chubby, GoogleBot, their indexing pipeline, the actual reverse index query execution engines, the search engine aggregators, Spanner, etc.

If you think about it, this is all perfectly sensible. Google sees back-end technology as a key competitive differentiator. Facebook needs to build back-end technology, too, but is not as vulnerable to a technologically equal competitor; if we were, Google would have made good on their promise to destroy us by now.

Disclaimer: I'm a Facebook engineer who contributes to the HipHop effort.