…that's not in person, is email. Here's why:
Email isn't based on a central system. If Facebook was to go down, or get hacked, no one using Facebook's messages as their main way to communicate could talk. Sure, Microsoft and Google (combined) run more than half of the active individual accounts (citation needed), but they don't have to be functional for one person to send a message to another.
- No strange technical requirements
There are so many different types devices that a person building a messaging SaaS would need to address that it would be difficult to build something that works everywhere. Sure, it'd be nice to have a desktop client for WhatsApp, but nope, they haven't made one. With email, it's a standardized protocol and anyone could build an email client for their own software environment if they wanted to.
- Offline access
If one has their email set up for offline access, they could have all their saved emails on their device and access them without an internet connection. This is especially important in areas with spotty cellular service, or in the event of a major catastrophe. One can even write an email, place it in their outbox, and know that their device will send it when the server becomes accessible.
Most email services offer encrypted access. Some (not Google!) go so far as to not even have access to your emails on their end. (Google, for example, does have access so that they can sell advertisements to you.) If you wanted to be even more secure, there's many ways you can integrate PGP encryption and signing in to your email for complete end-to-end security.
In my opinion, the biggest reason why email popularity is is waning is that all the popular ways of accessing it are built as generic email clients, serving a generic purpose. What I really hope to see down the road is clients for more specific uses, and if I had the skills, I would build it myself. How about an email client that works like SMS? You set it up with your server using IMAP, it creates a folder for all its content, and you're off to the races. A simple, short field, no formatting, and no subject line. It might run a little slower than a regular email client, but not by much!
Everyone has access to email; not everyone has a Facebook account. Every device can access email; not every device can run WhatsApp. Anyone can run an email server, impose their own rules, and not be effected by the rules or downtime of other email servers. It's really the online communication form that will last.