We use a different persona on each app
In the real world we behave one way in a meeting at the office and a different way at a bar with old friends. Each of our friend groups has different interests and inside jokes, and so we approach them with tailored messages. The content we produce on each app is different. Even our personas and the content we produce in different groups within the same app is sometimes different.
But Facebook has trained us to collapse our personality into one monolithic identity. That’s only 10 percent of the real you. The majority of the real you doesn’t have an outlet. That is where messengers (and other services) come into play. That is where the content you publish in each app reflects your persona for that app – the consummate professional on iMessage with co-workers and the jokester with college friends on Facebook Messenger.
We use multiple messengers because they reflect our diverse social circles, our various personas, and the variety of content we publish.