Facebook defines Storytellers as:
The number of people sharing stories about your page. These stories include liking your Page, posting to your Page’s Wall, liking, commenting on or sharing one of your Page posts, answering a Question you posted, RSVPing to one of your events, mentioning your Page, phototagging your Page or checking in at your Place.
In layman’s terms, Storytellers are users who make a tangible interaction with your Facebook Page. The term “Storytellers” refers to users who are creating “stories” or “objects” within their news feeds that may direct their friends to your Page.
This metric is important as it represents the total number of users interacting with your Page. You can increase this metric by creating more events, asking Questions, and increasing engagement per post. When creating Storytellers, you can also think of it as building Affinity with these same users. They are taking action that notifies Facebook they are building Affinity with your Page. Keeping an eye on Storytellers can provide useful insight.