And to close out the tetrafecta of Two Bit History posts (a, b, c), this pondering of FOAF is a great one to close it out upon.
The context within which I confused myself about having heard of FOAF, was Wordpress, which supports (within core!) a similar-but-not-really-that-similar standard called XFN, short for "XHTML Friends Network." It, likewise, appears to never have caught on.
Interesting to view ActivityPub in this light.