You have to keep in mind that you have to share and post a lot (!) when you take part in Facebook campaigns. So you are spamming others Facebook walls and a lot of your "friends" will unfollow you.
This.
And not only that but you ONLY get paid when the ICO finishes.
I believe most people willing to do those things are in countries where that money would really help them, waiting so much for a payment that is not sure(Depends on the Community Manager checking) is a big risk. Of course, there are some who use a lot of accounts - But that is another story.