The error is caused by Facebook cropping the full URL.  As a result, only the Campaign ID is listed in the URL. Because the full URL is not present, the user will receive an error page.

Alternative Solution: Insert the mailing's browser view URL (found through Link Manager of the content) into a provider that shortens the URL (such as, for example).  This will populate an abbreviated URL that you can insert into the Facebook code<shortenedurl>
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