Use the share verb when an actor recommends an object to others. The main difference between post and share is that in share the actor is not the owner of the object. For more information see: http://martin.atkins.me.uk/specs/activitystreams/activityschema#share
http://activitystrea.ms/schema/1.0/share
Because all of the object types are social objects at this point they can all be shared into the user's stream. Targets are not usually used with the Share verb.
Mary shared an article "10 Ideas for Great Mother's Day gifts under $100"
Sharing implies posting someone else's object.