Authenticity means staying true to your brand and what matters most to you. You need to have values and be passionate about what you are talking about or your audience will not be as inclined to follow what you say.

I would create social proof by having people use the products and seeing that every day people are using it. In order to create FOMO you need to create a campaign that makes the event or the product so good that you feel left out without it. This needs to be in everyone's hands and every where you look so that if you dont have it you really feel isolated from the trend.

If the comment was mean I do not think the brand should engage in it. However if it was about a negative experience I think I would reach out and understand where the root of the issue is, how the company can do better moving forward and if we can work with the customer to have a better experience in the future.