Perhaps it was not prioritized because a certain amount of scale, in terms of active members, is required to be very successful (to the extent of getting to "rule the world" kind of level)? Many companies start with fewer user hostile items until they reach a massive scale so as to protect their turf from competitors who may use that against them as well as to keep people coming back to the platform often and for long durations.

Given that FB is now a public company, it needs to prove to shareholders, beyond anything else, that it's growing its revenues and profits. Turn FB into a company that puts its users first instead of its profits, and its stock price would probably be 1/3 of what it is today.