They snagged a few mid to larger streamers a year or two ago, then once contracts ended alot of them immediately moved to youtube for a fatter contract at the time or back to twitch for the viewership/community for growth.
I think that this might have been a larger portion of it. A large amount of the audience for this type of content has either deleted Facebook, or is too young for Facebook. I completely stopped watching T90 (a very entertaining/iconic Age of Empires 2 caster) because he moved to Facebook, and several of my friends did too.