Help in a function

Hi, I need extract a data into a function but i cant.
My code is:

function fb_like_thumbnails()
{
global $posts;
$default = ‘https://www.vidalowcost.com/wp-content/themes/Elements/img/logo_face.png’;
$content = get_post_meta($post->ID, “cuadrado”, true);

if (is_single()) {
$thumb = $content;
}
else
$thumb = $default;

echo “<link rel=\”image_src\” href=\”$thumb\” />
}

The bold text is the problem don′t return nothing, and I need this value, because is my select image for the singles posts.

Can you helpme? thanks