I'd guess that the engineers who are building it think it's cool and want to talk about it. Facebook seems to be generally open about their internal systems, presumably because they don't see it as their competitive advantage (unlike, say, Google).
Google talks a fair bit about their internal systems at this level of "descriptions but not code" - Bigtable, MapReduce, Spanner, Flume, Chubby, and more have been influential.
In fact, they do more than just talk: they often publish papers describing how they work. The open source community has since recreated a lot of them, which has proven useful to a lot of people (e.g. HBase, Hadoop, Apache Crunch, etc.)
