[dotcoma]

(how) do they make money ?

[dexwiz]

By extorting companies to improve their score by removing bad reviews or promoting good ones.

[cheikhcheikh]

they list a bunch of stuff in thei paid features https://www.glassdoor.com/employers/features-and-pricing/

looks like you pay for advertisements of jobs, some more marketing features and some other crap

[dotcoma]

It doesn't sound like a great business to me, not to mention the risk someone else rightly mentioned here of being taken to court by companies.

[TillE]

It is extraordinarily difficult to win any kind of defamation lawsuit in the US. The bar is really high.

I certainly wouldn't try to run a business like that anywhere else in the world, but in America they're probably fine, with just some basic screening.