Yep, the demographics on this thread have changed since I posted. A lot of us read HackerNews, and maybe we contribute out of proportion to our numbers when we see FB getting slammed unilaterally, as was the case before I wrote my response.
We don't use mailing lists much at Facebook, preferring to use Facebook where possible. Like anybody else, Facebook employees who see something of interest on HN often use Facebook to share the link with those friends. There is nothing conspiratorial about this process; it's the same way all other links on Facebook spread through peer groups.