The 6 month priorities are due to PSC, Facebook's performance review cycle. Work that took more than a half effectively doesn't count for bonuses or promotions and could possibly get you fired, so it's disincentivized.
It also makes for shitty service quality around the end of June and December, as people rush to push things to prod. (I've heard this may have changed, but it's hard to notice good service quality).
(Former FB employee)