The National Labor Relations Board has recently upheld that posts on social media can be considered protected concerted activity. (Recent Cases, 34-CA-012576)
A negative company review on Glassdoor is discussing working conditions with an attempt to improve them (see "Advice to Management"). So long as there are no false claims in the review, and there are other active employees engaging in providing reviews for the company, would this be considered protected concerted activity as well?