Wordpress Post featured image URL in the header

To answer this one and point to the real problem:

As the <head> HTML tag comes far before the actual loop, you’ll need something else than the global $post.

get_queried_object();
get_queried_object_id();

The plugin

The code is tested and works.

As you might want to keep this functionality when switching themes, I’d suggest wrapping it up in a plugin.

So the actual plugin should be something around the following lines:

<?php
/** Plugin Name: (#70215) »kaiser« Post Thumbnail image for FB */
function wpse70215_fb_img()
{
    // Not on a single page or post? Stop here.
    if ( ! is_singular() )
        return;

    $post_ID = get_queried_object_id();

    // We got no thumbnail? Stop here.
    if ( ! has_post_thumbnail( $post_ID ) )
        return;

    // Get the Attachment ID
    $att_ID = get_post_thumbnail_id( $post_ID );

    // Get the Attachment
    $att    = wp_get_attachment_image_src( $att_ID );

    printf(
         '<link rel="image_src" href="https://wordpress.stackexchange.com/questions/70215/%s">'
        ,array_shift( $att )
    );
}
add_action( 'wp_head', 'wpse70215_fb_img' );

The plugin

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