Custom Facebook message using the_excerpt filter

Hello, folks.

I’m trying to achieve a custom message to be used at the publication of the post on the Facebook page.

I’m trying to use the_excerpt filter to get the first paragraph of the text, trought the following function:

function xyz_facebook_excerpt_format($excerpt)
{
    $content = get_the_content();
    $content = apply_filters('the_content', $content);
    $end_of_paragraph = strpos($content, '</p>');
    $excerpt = $end_of_paragraph ? substr($content, 0, $end_of_paragraph + 4) : $content;
    return $excerpt;
}
add_filter('the_excerpt', 'xyz_facebook_excerpt_format', 5);

The option to apply the_excertp filter is enabled at the plugin settings page, but the grabed paragraph is not being used as desired. Sometimes it was replaced by the standard first 50 charactes of the text.

I’m using the_excerpt filter with the priority 5, to trying to ensure that it be triggered in time.

Any help would be appreciated.