Excerpt Is Disappeared

you have forgotten to return the unaltered $content if this is not a single post:

function google_adsense($content) {     
  if(is_single())     
  {         
   global $post;         
   $thePostID = $post->ID;         
   $more_span = '<span id="more-' .$thePostID.'"></span>' ;         
   return str_replace($more_span, $more_span . " " . google_reklam_ekle(), $content);     
   } else { 
   return $content; 
   }
}