add_action( 'the_post', 'campaign_remove_nextpage', 99);

function campaign_remove_nextpage ( $post ) {

if (($_GET['utm_campaign']== 'Facebook' ||  $_GET['utm_campaign']== 'Twitter') && (false !== strpos( $post->post_content, '<!--nextpage-->' )) )
{
}
else  {
    $totalArticlesPages = substr_count($post->post_content, '<!--nextpage-->');
// Google: not paginated
// Direct: not paginated
// Camp: paginated

    // Reset the global $pages:
    $GLOBALS['pages']     = [ $post->post_content ];

    // Reset the global $numpages:
    $GLOBALS['numpages']  = 0;

   // Enable multipage
    $GLOBALS['multipage'] = false;
 }};