FusionAuth is a multi-tenant authentication service used by John Maia and his team at Seegno, which handles over five thousand tenants in their largest environment. They use FusionAuth to manage users, session tokens, social sign-on, and offer features such as passwordless login and email services. The solution has saved them significant time and money compared to maintaining their own authentication solution. John and his team have found FusionAuth to be the most versatile, easy to use, and configurable solution, with a quick response from the support team when they encounter issues, allowing them to find solutions together. However, they do struggle with running multiple instances of FusionAuth in Kubernetes, requiring workarounds such as creating separate deployments and services to achieve this functionality.