You've explained where Facebook's likely interest lies, but not why that feature is actually popular.

I don't see any difference with how HN and Reddit attract people. They all seek the added social aspect to reading news; and Facebook is just what people know.

>Apologies for being cynical, but I'm not surprised.

Luckily cynicism doesn't equal realism. FF devs often make data-driven decisions, and none of the devs showed interest fixing all the accumulated bugs. So they left this to matter to add-ons, the big button was eventually stored away and the one in the address bar abandoned for Pocket. No sneaky marketing strategies involved.