There are many 3D virtual world libraries and engines that claim to be high performance and highly scalable, often they tout being able to scale to 100's of simultaneous users per-server, this is basically nothing. These solutions fail because they use the classic client/server model and centralized database. Take for example EveOnline, they have one of the most optimized MMO systems, yet even with huge amounts of RAM and a SSD backed SQL database, they only scale to tens of thousands of concurrent users.
The Solution - Peer2Peer Networking
The solution to scale without limits is pretty simple, drop the client/server model and use the peer2peer model like bittorrent. I was surprised to find only a few projects have tried to use this technique: Solipsis, VAST, and NICTA's research project.
Voronoi Overlay Network
Vast has a smart solution to manage communication between peers that can do spatial queries in the same amount of time regardless of how many peers are connect to the network. This amazing research was pioneered primarily by Shun Yun Hu starting way back in 2004. Do not miss reading his research papers.