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Minecraft is not an MMO. However, it's on this site, under the category of "MMO". I consider this more of a clerical error than anything, but whatever. It's here.
I've played on a Minecraft server with 150 concurrent players almost reliably. Aside from the total number of players, it was a really fun, open world, sandbox, ffa PvP MMORPG experience. :-) Now I'm running a Minecraft server modded with Feed the Beast for myself and some friends, which is a lot of fun, but I don't think a server exists that could run the Feed the Beast mods for 150 people.
So how about you? What's the closest you've gotten to the MMORPG experience playing Minecraft? What's the largest number of concurrent players you've been on a server with?
I can not remember winning or losing a single debate on the internet.