Postmodem depression is the unpleasant feeling you get when you haven't had access to the Internet (i.e. QQ, Weibo and WeChat) for a "long" time, like several minutes. You don't have appetite, not in the mood to enjoy beautiful scenery or talk to people, the only thing you care about is where you can find a WIFI hotspot.
Mom: Johnny, come to dinner!
Johnny: Not hungry!
Mom (to dad): What the hell is wrong with him?
Dad: He's got postmodem depression; he can't update his WeChat Moments.
Mom: But we have only been here for 15 minutes.