Glassdoor seems ripe for astroturfing. At my last company, HR specifically targeted new hires (they didn't want people that had already been there and been soured by the culture) in a mass email to please leave a positive review on Glassdoor. It's also just painfully obvious from looking at the reviews which ones are fake, and it's a lot of them.

This certainly isn't true for all companies, but Glassdoor isn't exactly immune to shilling.