Customers will always find ways to communicate with each other, especially when they feel they've been wronged. No company should ignore this expression of dissatisfaction completely, especially if they have active online customer services. If your customer base can vent their frustrations on the same channel as the one you pay attention to, it might result in some hard but important truths.
But even those sites that aren't a priority can yield important information. While it's unlikely that your business is overly aware of the website Reddit, for example, its users recently listed their least favorite companies, after receiving a prompt from The Daily Beast. The news source reported on the results, which might be seen as a litmus test of some very basic, outstanding service concerns.
Some of the businesses listed were Best Buy, cited for its apparent lack of relevance in comparison to other internet-based retailers, and having workers that were unable to answer questions, and British Airways, which is perceived as being insensitive toward multiple customers due to circumstances beyond its control. Unnecessarily high or unfair prices were another source of complaint, at least for businesses like Adobe and AT&T, as well as some accusations of inhumane practices.
Given the expletives and free language that might accompany such opinions (with multiple being labeled as "evil"), it's clear that these networks may not always be seen as workable sources of criticism. However, a web-savvy company might pick up on these complaints and work some customer-facing solutions in through other means, including live calls, that can dissuade against feelings of dissatisfaction.