So did switching to their homebrew serialization format -- in fact, most of the article is about how they managed the changes (which touched codebases at multiple sites in a fairly large organization).
Those switches all occurred at the pipeline level, leaving the map-reduce platform untouched. Switching our base logs to something like Parquet, Thrift or Protobuf would be a much larger project. We do support writing and reading Parquet to allow us to interface with other big data systems.