Hem, isn't it the "social dependence issue" ? Immediacy of response is not a sign of interest for you or the subject at any time... And not responding in the second is not necessarily a way people communicate (thanks to those marvellous tools like email, FB, post mail, fax... to let us think of what to answer). And you are not alone to question yourself... http://www.web-strategist.com/blog/2007/11/30/a...
Ther same thing is happening in France : the CNIL (National Information Liberties Commitee) is questionning Facebook on their use of personnal datas... More here (in french, sorry) http://www.cnil.fr/index.php?id=2383