The short answer is yes, you can get keloids from piercings. Keloids are an overgrowth of scar tissue surrounding the piercing, and they can range in size; some can be quite large. It is important to remember, though, that not all growths surrounding a piercing are considered to be keloids. Infections of the skin can look like raised bumps, and sebaceous cysts can also look like scar tissue, but are something else entirely. Some people are simply more prone to keloids from piercings, and these people should generally just avoid getting pierced together. Otherwise, there are a few methods that may help to prevent scar tissue from building up around a new piercing.
Once keloids from piercings, or from other trauma to the body, such as surgery, have started to appear, it is typically too late. Keloids typically need to be surgically removed, and it is largely unknown if they will reappear since the cause seems to be primarily genetic. People of African American descent are much more prone to developing keloids from piercings, for example, than some other races. Steps taken to care for a piercing or tattoo may or may not actually prevent keloids from forming, but it is worth it to try.
It is possible that the friction caused by the piercing rubbing against the skin can contribute to the growth of scar tissue, as well as pulling or tearing the piercing soon after it occurs. Being careful with a new piercing, and taking care not to sleep on it, or consistently pull it, may help to prevent scar tissue from forming. In addition, cleaning the piercing as recommended by the piercer, generally with alcohol or saline rinses, can help to prevent infections. Infections can lead to scar tissue growth as well, and can potentially be dangerous.
One other important thing to remember is that even if keloids from piercings haven't developed in the past, that doesn't mean they can't still develop in the future. Certain areas of the body, on some people, may be more prone to these growths than others. If a keloid seems to be growing around a new piercing, it is a good idea to visit a dermatologist who can assess the situation, and determine if treatment is needed. In many cases it might be necessary to remove the piercing and receive treatment in order to prevent the keloid from getting any larger.