This is my concern as well. It may be what will naturally be though (yin-yang and all that), and thankfully there are ways to get eyes into a bubble even if it's protected to get intelligence on what violent activities may be attempting to be rallied. The interesting and perhaps other fortunate piece to these communities being online is that everyone is quickly mobile (unless under strict watch and censorship locally through physical communication channels) - and so people are less stuck in a community if they realize it is a bubble - and are also more easily able to be reached. I just posted more related thoughts in a comment here - https://news.ycombinator.com/item?id=18213574