4. Negative posts, tweets, and comments
Brands develop social media platforms to establish dialogue and connect with consumers, but it can sometimes end up as a platform for abuse or negativity. While it may be painful to see negative feedback about your brand, it can also be highly instructional, informative, and valuable. However, if negativity encroaches outside of the brand, it can deflect from the objectives of the effort and project negatively on your brand.
Skittles recently launched a site integrating multiple social applications including Twitter, Facebook, YouTube, and Wikipedia, in a gesture to allow consumers to define the brand. Users started posting racial slurs, profanity, and other inappropriate comments. Skittles apparently decided to allow the community to manage itself (or not) as illustrated in the comment below from "Summerstar1227."
I think people are blowing this out of proportion. Skittles is a candyโฆ usually marketed towards kids. Why do adults feel the need to go on a website mostly directed towards children and swear and say rude things? Grow up already! More like freedom of ignorance, not speech.
Nevertheless, many pundits were quick to condemn it as a social media "failure." Skittles' seeming decision not to filter or respond directly is not necessarily a failure, but it does illustrate to brands the importance of monitoring these platforms and being prepared to respond in a way that aligns with a brand's image and objectives.

Lesson learned
What one might consider a failure may be a success to someone else, depending on the desired outcome. Decisions about how to respond (or not), filter, participate, or moderate should be made based on consideration of how each approach would reflect the brand persona and objectives of the effort.
The degrees of success or failure will also differ as viewed by distinctly different stakeholders and constituents. It is impossible to determine failure without clearly defined criteria for success. The failure occurs when you don't know your options, consider your constituents, and plan upfront accordingly.