In this guide:
Explicitly shared actions let apps notify us when people want to share something prominently, like they would on Facebook. You mark Open Graph actions as explicitly shared by adding the fb:explicitly_shared parameter when publishing actions. For example, when people choose to share a run they took with a fitness app, they expect the content to appear as if they had posted it directly to Facebook.
The use of this parameter drives more traffic to apps that people use to proactively share content to Facebook, but it doesn’t penalize other types of apps. Explicitly shared actions are eligible to appear as stand-alone stories in the news feed, and they'll appear consistently on the left side of the timeline. Explicitly shared stories will always be shared in the news feed of the person who shares them. All other activity will continue to appear in ticker, trending units and collections in news feed and in recent activity and collections on timeline.
Use explicit sharing in cases where a person makes a conscious decision to share an experience back to Facebook in real time. It represents something they would have entered or uploaded into the Facebook composer for sharing.
Explicit sharing can happen through a variety of actions, like posting a photo or adding a message to an action they've taken. The important thing is that people clearly understand they're posting this action to Facebook at that time and that they have an in-line way to opt out.
Here's an example of an app explicitly sharing content on Facebook:

There are some activities that happen naturally in the flow of your app that you may want to publish but which people may not want to highlight explicitly on the timeline. These could be actions like listening to a song or reading an article. In these cases, omit the fb:explicitly_shared parameter (preferred), or optionally mark the actions as implicit by setting this parameter to false.
The following types of actions should not generally be labeled as explicitly shared, even though they may be explicitly initiated by a person using your app:
When people take these actions, they may still want to share the activity back to Facebook. The generated story is more about defining a person's identity rather than looking for an immediate social interaction with friends.
With the fb:explicitly_shared parameter, you can tell Facebook the type of action generated by someone using your app. Passing true indicates it to be an explicit action and lets it appear on the left-hand side of that person's timeline. Passing false or leaving off the parameter completely indicates to Facebook that the person did not wish to explicitly share on the timeline (that is, they intended this action to be an implicit activity).
NOTE: All activity will be considered implicit if fb:explicitly_shared is not set to true. For example:
curl -X POST https://graph.facebook.com/me/nyccookbook:cook \
-F "access_token=<valid access_token>" \
-F "recipe=http://www.sugarmedia.com/nyccookbook/cookie.html" \
-F "fb:explicitly_shared=true"
Note that there may be scenarios when a specific action is sometimes explicitly shared and other times when it's not. You should only include fb:explicitly_shared=true in the instances where a person using your app clearly intends to share activity on Facebook.
Facebook treats explicit actions similarly to status updates that people make through the composer on facebook.com. This means that these actions are eligible, but not guaranteed, to be shown as standalone stories in their friends' news feeds. (Facebook always shows explicitly shared stories in a person's own news feed and on the timeline as described below.)
Facebook displays both explicit and implicit actions in the recent activity box and collections on timeline.
For any story posted with fb:explicitly_shared=true, the story appears on the left-hand side of the timeline as a single story. There will be no other visible changes.

During development, you can use the explicit action parameter without submitting your action for approval. To use explicit actions in production, you must first define your action as an explicit action in the Action Type definition screen and submit your action for approval. Please review the Open Graph Submission Process to help ensure your action will be approved.