Having been involved in various online activist forums including high-profile Anonymous and Occupy groups as well as low-profile environmentalist and political theater groups, I'm pretty sure this kind of thing has been going on for a while and not just for supposed terrorists groups. And sadly it can be very effective. When an online forum has dedicated professional accounts that post 24 hours a day 7 days a week (meaning there are multiple people operating them) and they are very familiar with all the internal group dynamics and personalities, and they use multiple demoralizing techniques from subtle misinformation to playing off different personalities to crass attacks, the effects can really destroy the cohesion of a group and drive people away. It's a struggle figuring out how to combat this.