They don't get into it, but the AIM host infrastructure was based on the event-based non-blocking libraries written by Lippke. The host complex was a distributed hash table (routed on relevant data, e.g. Screen name, ICQ number), with the extra step that you can migrate the in-memory state from one host to another while still taking incoming work. Quite a powerful infrastructure to build highly-available services. On…
ICQ (the original chat client) was around for a full year before AIM launched.