Unfortunately, this data is not available. What is possible through Link Graph API objects and the FQL link simply retrieves the given instance in which the link was shared by the user. You can get the number of messages you like on the link posting page, but this is most likely not what you want.
I discussed this 3-6 months ago with the guys on Facebook, and then the consensus was also that getting this data through the API is impossible. As far as I know, things have not changed, and there are some privacy arguments for storing this data from the API.
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