Purely anecdotal, but I worked for a small software company that were truly awful. I left a review on leaving that was honest, but factual. Weeks later, a competing review was left up that countered all of my points as positives - often hilariously so, shit like "we don't unit test because we aim to be right first time".

My review was then removed from Glassdoor. Knowing the rumours, I used an old eMail address that would be hard to link back to me. A little while after, the COO complained on LinkedIn about fake reviews on Glassdoor, pasting my email address that I had used to register.

I trust the negative reviews on that site, but not the positive ones. I also don't trust Glassdoor as a company to do what it says it'll do...