The world record for number of players in a single game shard (and visible to each other!) has been officially broken (official meaning that Guinness World Records has verified it)! Man vs Machine browser-based FPS prototype developed by MuchDifferent supported a one-off event of 999 players.
The event was made possible by their custom server backend, Pikkoserver. The client software was a Unity browser plugin. The server ran on eight server threads on what appears to be a single machine. Clients would have need 256 kbps modem. There are videos to demonstrate that it did function mostly well and certainly showed a lot of action (though the language of the average FPS player hasn't improved much since I gave up playing a certain FPS a few years ago!) I wonder if they'll publish the event logs so that people can analyse the fairness?