Well remember that this was a few years back so things were a lot different from today.
1. gchat (or "Google Talk" I think?) was a nice simple app that was replaced by a rebranded one that had different functionality and a UI that was either different or flakey/slow/unreliable (I forget all the details). I later learned that actually we were all duped by google - gchat itself was only turned off completely very recently, but at the time they managed to convince us that we had to use Hangouts instead. In any case, the whole thing was annoying enough that we left because ...
2. Whatsapp was starting to be very widespread and was a natural replacement for us as most people either had it or could quickly get it (you just needed a phone number and nearly any sort of phone, many feature phones support it) and it had a simple chat-only interface.
3. FB Messenger was not a separate application at the time, it was integrated in the FB app iirc and you still had to be a member of FB and add any contacts as "friends" to communicate. At the time it wasn't seen as a general-purpose chat app by _anyone_, just a way to talk to your FB contacts.
Hope that makes sense. If you're curious about the gchat->hangouts switch, I'm sure a quick search will turn up a few disgruntled articles - there were quite a few people who were annoyed. It probably didn't help that Google were in the habit of dropping products, so a few people may have assumed that they'd do the same with the chat app.