I wouldn't exactly call it a huge industry, it's been shrinking for years as indie devs shifted focus to mobile and steam. FGL.com the main marketplace for monetizing flash games has scheduled shutdowns [1] of all their flash services to focus on their mobile services, they also announced their ad revenue as one of very few providers was collectively down to $50/day [2] when they announced that was shutting down last month.
[1] https://www.fgl.com/blog/2015/11/fgl-to-drop-support-of-cert...
[2] https://www.fgl.com/blog/2015/11/fgl-will-no-longer-support-...