Instagram is huge among JW kids (it least it was 3 years ago) because they were warned off other social media sites like Facebook.
Instagram gave them tighter controls on who friended them and can see the information.
I am not sure whether JW kids would be brave enough to show any apostate stuff in their feed because word would soon spread, particularly as parents and other JW congregation members would be able to see the information. It wouldn't take long before the whistle was blown.
When I left, some were experimenting with Snapchat but that has gone mainstream too.
Of course, I am old and out of the loop now, so I am probably wrong lol