fadtivist
A fadtivist is a phony who takes credit for helping people, but only when the cause is trendy and makes them look good.
"I’m a fadtivist because I only help people when it’s popular!", @FadtivistMan
"That fadtivist is full of hot air!", @RealHelper2
"I’m not a fadtivist, I’m a real person with a fake cause!", @FakeCauseFan