With more ways than ever to communicate and interact online, people are naturally going to use such platforms to air their grievances - especially with brands, and what they might perceive to be poor service, low quality, etc.
This is why word of mouth is now so crucial - in the past, word of mouth was limited to those you spoke to or saw, to your immediate social circles of family and friends with whom you maintained regular contact. But now, you have access to the various perspectives of many, many more people, some of whom you might not even know personally. And even if you don't have an 'in real life relationship', their opinions can be highly influential in what you're willing to spend your money on.
That's also why, for brands, dealing with posted complaints and issues is a must. Every digital marketing strategy should have some level of planning built-in for how you're going to deal with negative comments and reviews online - because if you don't, you're missing out on valuable feedback, and the opportunity to improve brand perception, and build your digital presence.
If you're not sure how you should be going about dealing with such, this infographic from Media Update will help. The listing covers off on some of the key considerations of brand response to negative online mentions. And while we would advise on some additional caution when using the last point ('Respond with Humor'), there are some solid notes and pointers in here to help you establish your approach.