OK, this extract works for 1.20
function JidanniOpenGraphMetaPageHook(&$out,&$sk){global $wgLogo;
$out->addHeadItem("meta:property:og:image",
Html::element('meta',array('property'=>'og:image','content'=>wfExpandUrl($wgLogo)))."\n");return
true;}
$wgHooks['BeforePageDisplay'][] = 'JidanniOpenGraphMetaPageHook';