It also doesn't make sense because terrorists would obviously be using encrypted channels primarily, not some shabby public plain text forum. And most organization would be on Facebook, with orders of magnitude more users and better tools.
It is well documented that ISIS recruited people through Facebook and Twitter, and even YouTube.
Remember: before the terrorists get those encrypted apps, they need to be radicalized first. That step is done on public social networks, where network effects help spread their message to a receptive audience.