Obrigado pelas respostas.
kongming, sim, e o plugin "Tweet, Like, Plusone" adiciona as "meta property" necessárias.
Pedro Lopes, o resultado da análise do site é o seguinte:
"
Critical Errors That Must Be Fixed
Bad Response Code: URL returned a bad HTTP response code.
Errors That Must Be Fixed
Missing Required Property: The 'og:url' property is required, but not present.
Missing Required Property: The 'og:type' property is required, but not present.
Missing Required Property: The 'og:title' property is required, but not present.
Open Graph Warnings That Should Be Fixed
Inferred Property: The 'og:url' property should be explicitly provided, even if a value can be inferred from other tags.
Inferred Property: The 'og:title' property should be explicitly provided, even if a value can be inferred from other tags.
"
Não percebo o porquê dos erros. No código fonte aparecem estas propriedades, que tal como referi são inseridas pelo plugin "Tweet, Like, Plusone", isto é:
<!-- This site is powered by Tweet, Like, Plusone and Share Plugin -
http://letusbuzz.com/tweet-share-like-plusone/ -->
<meta property="og:type" content="article" />
<meta property="og:title" content="TÍTULO" />
<meta property="og:url" content="URL"/>
<meta property="og:site_name" content="NOME DO SITE" />
<meta property="og:description" content=" DESCRIÇÃO "/>
<meta property="og:image" content="URL DA IMAGEM DE DESTAQUE" />
E o que significa o erro "Bad Response Code: URL returned a bad HTTP response code"?