I was thinking recently regarding various communication methods we use. Most people I know use at least three chat apps on their phones. All of them use email. Having at least three active social media accounts is a norm. The notion of a norm, by the way, should not be discarded — a person may be seen as weird if they don't have a Facebook account (because perhaps they have something to hide). I think that calendar is also a communication tool, which is one of my favorites because it binds with our perception of time.
Every successful app or a service should eventually become a chat application in some shape or form. I think the ones that do it well are those that augment the conversation in some way, and not necessary in a clever bot-like way. For example, if you're discussing a schedule, the app can give you a shortcut in its interface to reference time slots, and show to all of the people if one is busy. Or if you're working on a website design, the service might have a 1-click option to put a screenshot of the website right in the conversation, so that when you say "How do you like this new button?" the screenshot will be right under this message. If the service allows to augment the conversation with such references, you can without much hassle turn this conversation into an email thread, and so your clients don't need to use the app, but you can also keep the conversations bound to their original context for future reference.
Email is so far the safest calm haven for a distracted mind.