I think it’s perhaps a wider trend on Facebook. In fact, is Facebook in general going out of fashion? I think there are growing signs it’s past its peak.
What I have noticed anecdotally is that kids don’t seem to use Facebook as much. It seems to be becoming an older person’s platform. I think kids use others, such as Instagram etc.
Often there are semi official WhatsApp groups maintained by group elders for spreading official instructions and information. (Such as meeting times, and basic health news of members)
I personally don’t know of JWs using Whatsapp, but it would not surprise me. For some reason, Whatsapp doesn’t seem to be as popular in Australia as in other countries (although I personally think Whatsapp is a fantastic app). I think it is seen a bit as an encrypted communication app that drug dealers would use.