I really don't believe that Facebook's developers are incompetent. And it's quite easy to neglect work when there's a business case for leaving that work undone.

I suspect that as with any large enough organisation there is a spectrum of competence across their workers, raging from high to low. I obviously have no proof that there isn't some big conspiracy afoot, but given the choice to bet on conspiracy or incompetence as the primary factor for the issues we're seeing, I know what I'd put my money on.