I am a bit confused of what kind of relationship you have with him at the moment ? Im guessing youre friends with him and talk a lot on facebook but not in real life? He might not be responding as he is busy or he may see you as a bit obsessive or annoying , even thought you don't realise It , just don't be too obsessive and say hi every hour or minute his on but be nice and be casual and ask what hes been up too that he hasn't been online but don't dwell on it too much he may think youre wanting to know his personal life