Trevor Robinson, co-founder and CTO of FITT Finder, discusses how his startup, a search engine for health and fitness activities, utilizes FusionAuth for user authentication. FusionAuth was chosen to replace custom user management due to its scalability, security, and cost-effectiveness, especially important for a bootstrapped startup aiming to scale to millions of users. FITT Finder leverages FusionAuth's OAuth, login, and registration APIs to manage user credentials and authentication, with plans to incorporate social sign-on and SAML SSO. Despite minor concerns regarding zero-downtime upgrades and memory demands, Robinson appreciates FusionAuth's core functionality, API, and community support, running the service on AWS ECS with MySQL as the database. The integration of FusionAuth saves significant development time and ensures adherence to evolving security best practices, allowing FITT Finder to focus on maintaining up-to-date activity data through periodic jobs and third-party APIs.