Chat free no nothing
This is something that few chat sites would even consider to undertake.
The secret for going from zero to seventy million users overnight is to avoid doing it all in one fell swoop.
Fault tolerance is a desirable characteristic of any big system: if an error happens, the system should try its best to recover without human intervention before giving up and informing the user.
The results of inevitable programming bugs, hardware failures, et al., should be hidden from the user as much as possible and isolated from the rest of the system.
Surfacing connected users' idleness greatly enhances the chat user experience but further compounds the problem of keeping presence information up-to-date.
Each Facebook Chat user now needs to be notified whenever one of his/her friends (a) takes an action such as sending a chat message or loads a Facebook page (if tracking idleness via a last-active timestamp) or (b) transitions between idleness states (if representing idleness as a state machine with states like "idle-for-1-minute", "idle-for-2-minutes", "idle-for-5-minutes", "idle-for-10-minutes", etc.).