Example: Everyone who even knows someone who makes $500K at Facebook seems to love posting about it here, but nobody who makes $100K there seems to feel the need to boast about it.
There are tons of low level employees posting their salaries. If you work in those roles, that data could be more valuable to a candidate than Glassdoor's amorphous projections.
I think the user behavior that facilitates this is the same behavior you see represented on Blind.
https://www.levels.fyi/offerings/
They also have users upload W2s, offer letters, and supporting documents in addition to proof of identity.
https://www.levels.fyi/verified/
They have their own job advertisement business and they partner with third party recruiting companies to get anonymous data about offers candidates receive.
They also just take having accurate compensation information seriously so they do things like outlier and forgery detection as well as seeming to have a much better algorithm for processing the information they receive.