Second Life involves a lot of per-user message sending. IM, group chats, friend requests, marriage proposals, dialogs with in-game merchant systems, payment notifications, persistent announcements, and a lot that I'm forgetting. They're all roughly the same thing -- messages that need to be delivered to either a specific user or a group of users, with flags for whether they get discarded when the user is offline or whether the user has to explicitly acknowledge them.

It was probably overreaching for us to try to find a system that would handle all this for every user. But even hundreds of thousands of current users, the bare minimum we could expect to be concurrent, were not well supported by the existing solutions.