There is a common rule – things that could be made by everyone, cost little. Tasks in Facebook and Twitter bounties are rather simple and convenient even for newbies, so there are a lot of participants, and it decreases the stakes. And vice versa – you need special skills and knowledge for creating a video or writing the article, so I haven’t seen such amount of participants in these campaigns, and they also are highly paid, as well as signature bounty.
It seems to me that the Amazix have read this post and their decision to cancel Facebook and Twitter bounties has something to do with it))
Bounty hunters, who participate in various content creation campaigns, receive more rewards. They spend more time on such work than participating in social networks. But if the ICO turns out to be a Scam, then their disappointment will be greater also.