A new research report by Verto Analytics shows use of Siri is down 15 percent over last year, but the personal assistant is still twice as popular as its nearest competitor. Siri’s 41.4 million monthly active users still dwarfs Samsung’s S Voice at 24.8 million, but last year Siri boasted 48.7 million users. Other services, like Amazon’s Alexa and Microsoft’s Cortana, are growing steadily because of their ties to the rising…