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Any page where Facebook meta tags are included does not pass the HTML5 validation.
<meta property="og:type" content="website">
property
is not valid attribute in HTML5.
Solution is to rename property to name.
Second problem then is og:type
is not valid as a value of name.
This is not Drupal related problem nor Addthis or Sharemessage problem, this is in general Facebook and HTML5 problem.
I am adding this issue here because Sharemessage code should be updated as soon as the solution shows up.