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.