Many 'creepypasta' stories are shared widely. This constant exposure can desensitize some people, but for others, it can embed the fear even deeper. The more they hear about these horror tales, the more they start to believe that such terrifying things could actually happen in their own lives.
They can heighten it. For example, the vivid and often grotesque descriptions in these stories can make people more aware of potential threats in the dark or in isolated places. After reading about 'Jeff the Killer', one might be more scared of noises at night.