Home is exactly like every other modern MMO, without a standard combat system. It's unique feature is that it's combat system shifts depending on which game instance you are currently in. You've got a little avatar that you dink around with and make dance. You have a crap chat interface. You have micro-transactions to dress your little in-game doll up. You have a hundred+ people on a server instance. You have a home that features persistence. It is exactly an MMO.
Yes, it is a terrible game that has spent three years in beta, but pretending that it isn't an MMO is ridiculous.
If Home isn't an MMO because it doesn't have "fantasy RPG" tacked on to the end of that, then neither is Second Life. Or Huxley. Or Firefall. Or Secret World. Or Guild Wars. Or an endless list of others that if you try to argue they aren't MMOs you are rightfully laughed at for even putting forth the silly idea. "MMO" does not imply elves and dice rolls, or even large numbers of players in instanced combat as Guild Wars shows.
If we want to define "Massively", I'd say anything over the standard 64-player cap that you see in the upper end of shooters like Frontlines. MAG is borderline MMOFPS, where it does have level progression, it does feature 250 player servers, but it severely lacks in persistence and non-combat interaction.