They can't be, deciding which messaging app to use to contact a given person and have a reasonable expectation of getting a reply reasonably quickly, is very much an unsolved problem that everyone suffers from, whether techie or not. And it varies by country & subculture, e.g. over here in the UK almost everyone I know uses WhatsApp as their primary IM, whereas my understanding is that it's much less popular in the USA. And let's not talk about iMessage, the most closed of all IM platforms.