Meta's Muse AI assistant has topped app store charts, driving the company's stock up about 20% since its launch a month ago, marking its best September performance since 2022. While currently free, Meta is exploring business models for its personal AI agent, including monthly subscription plans for $20 or $100. The company faces challenges in user retention and trust, particularly after a recent nearly $17 billion settlement related to child mental health harms. Meta aims for Muse to be an anchor app, competing with rivals like OpenAI and Google, and leveraging its advertising business to subsidize long-term user adoption.