I've only noticed the 140 character countdown when updating your own status. I think that on any reply box that has a max/min character count, there should be a countdown/counter. Otherwise it's easy for a user to type too much and be faced with an error. They then need to either paste their reply in multiple replies or start all over again or edit down the reply, which can be a pain depending on how much they went over. It would be nice to have it when posting on someone elses profile, when responding to a status update or profile post and also when leaving reviews.