Building a presence system: who's online, and how to know at scale?

we're building a slack-like presence system, needing to know who's online and propagate those changes quickly to hundreds of thousands of users. the core challenge is balancing real-time accuracy with scalability and dealing with all the edge cases. think about multiple tabs or devices for a single user, flaky mobile connections causing 'flickering' online statuses, and how to scale the backend to support 500k+ concurrent users reporting presence updates. propagating these changes efficiently without overwhelming clients or the network is another beast. what architectural patterns have proven effective for highly scalable presence systems? how do you handle state reconciliation when connections drop and reappear? any specific technologies that have made this easier or harder?