No matter what business you are in - selling shirts or puppies or building a social network - you're eventually going to run into users who you've upset. Angry users are a reality of business, and you have to deal with them as you would any other problem. Just like you have to plan for the year's budget or for your computing capacity, you also need to have a plan to deal with upset angry users.
The first rule of dealing with upset users is to not panic, get emotional or angry yourself. As I hope to demonstrate below, angry users represent a tremendous opportunity that can actually be beneficial to your company.
1. Give them a channel to vent
The first thing that your plan needs is to have a channel where users can come and complaint/give feedback or just vent. This can be anything from a phone helpline, forums or even a feedback form on your website. The key, though, is that this channel has to belong to YOU. In an absence of an official channel for complaints, users tend to take out their anger on twitter or facebook or worse, in person to their friends. If they have a way to express their anger directly to you, at least you have the chance to know what they are upset about.
Some companies do this better than others. If you are not happy with your new car, there is no obvious way to vent your anger - other than to call up the dealer and yell at him. The car manufacturer may never find out about it. Amazon, on the other hand, has a "Returns Are Easy!" section which immediately has a calming effect if you're upset with whatever you bought on Amazon.
Once you have a channel for users to reach out to you, you have to ensure that you are listening. Your users have to know that you've heard their complaint and are doing something about it. If you're an e-commerce website, this means providing tracking information about the user's refund or return. If the user sent you a feedback form, you need to respond within some timeframe with an update - And not just an automatic email saying "We've received your feedback!"
The CEO of a well known hotel & resort chain apparently says that "Listening to customer complaints" is actually one of the services his resorts should provide to their customers. So, when a customer comes up to the front desk and complains that the "Trees are making too much rustling noise", the reception is expected to take out a "complaint form" and write down the complaint: Was the rustling noise "loud" or "too loud"? Was it consistent or intermittent? Would they like their room changed to a non-tree facing room?
3. Convert them!
Angry users represent a true opportunity for your company. The reason I like angry users is that at least they're passionate enough about your product/service to get angry. The worst case scenario is an apathetic user who has given up on your product and never comes back.
A business traveler once came to the same hotel & resort chain and found the hotel he was supposed to stay at to be overbooked. Just as he was getting upset, the CEO of the company was walking by, found out what had happened, invited the traveler to stay over at his OWN house for the night, and made him pancakes the next morning for breakfast!
Turns out, the traveler was a Harvard professor, loved the attention he got from the CEO and wrote a case study about it which is now taught to tens of thousands of students around the world every year.
If you pay attention to resolving the users' problem, going out of your way to help the user and doing whatever you can to win over the user, there's a better-than-normal chance that the angry and passionate user will turn into an advocate for your business. This should be the ultimate goal of your plan. Having users be passionate advocates for your business is the best kind of social marketing you can ever hope to get!