I could not wait to end university, so that I can stop using big tech products. Unfortunately people only use products they are familiar with. If they are always on facebook, they use that for groups. Good luck finding a local community uses anything but facebook groups.
Edit: at university, many pieces of info reaches you informally. If you are not talking to classmates, you might miss important news as well.
That explains it, then. Here it costs around 11 US cents per message. So if you are messaging a lot, they will add up at the end of the billing period.