As others have pointed out, growing any sort of two sided marketplace (in this case with companies and applicants) is really, really hard. On top of that, the quality of the reviews itself is very hard to regulate because Glassdoor isn't necessarily incentivized to promote the "truth" - they make money from the companies on their platform, not the applicants.
See related discussion recently: https://news.ycombinator.com/item?id=36634529