How We Built an AI Second Brain for 60K Knowledge Workers

Meta built an AI agent system internally called the AI Second Brain that now has over 63,000 installs and ~10,000 daily active users across engineering, PM, design, legal, finance, comms, and sales, growing from zero in roughly three months after a non-technical PM's adoption post. The architecture pairs Tiago Forte's PARA folder framework (Projects, Areas, Resources, Archives) with a root CLAUDE.md plus per-project CLAUDE.md files for progressive disclosure, an infrastructure layer of internal MCP servers and CLIs that give the agent scoped authenticated access to docs, meeting transcripts, task trackers, and code review, and a library of community-written skills as plain Markdown for workflows.The post credits four lessons: invest in the tool-access infrastructure layer before applications, prefer progressive disclosure over context dumping, low-friction bootstrap drives viral adoption, and composable Markdown skills turned the plugin into a platform that users extended themselves.