Here's the latest demo video of the Quark engine from MetaGravity, which employs "causal partitioning" (more on that below) to enable single shard virtual worlds with massive concurrency. They're showing off that technology later this year with their own Minecraft server, which they say will be able to handle up 100,000 concurrent players. (You can sign up here to request early access.)
Matthew Chuen, Head of Ecosystem at MetaGravity, says they're also able to handle high local concurrency, i.e. the number of avatars in your immediate vicinity:
"We have successfully handled 5,000 players within a 100-square-meter area with excellent performance," he says. "This is possible because our Causal Partitioning technology is fundamentally more efficient than traditional servers, avoiding the exponential lag that typically makes such high-density scenarios impossible."
Without diving too deeply into the technical details, here's how MetaGravity describes Casual Partitioning: