In response to Lum and Cuppy and Moorgard:

Online games and virtual worlds are not the same thing. If you’re building an MMO, you better be sure which one you intend to make.

Online game: a game played using network connectivity.

Virtual world: a simulation of space. cf long usage in the VR community, 3d viz community, etc. Somewhat deprecated since these days, everyone wants connectivity.

Online virtual world: a simulation of persistent space connected to via a network, wherein users are represented by proxies often terms avatars. Note thatboth “online” and “virtual” are often elided from the term.
Synthetic world: see online virtual world.

MUD: old term for online virtual world.

Avatar: the proxy by which a user is represented in an online virtual world.

Toon: see avatar.

MMOG: an online virtual world wherein embedded systems layered atop the spatial simulation present game rules.

MMORPG: an MMOG where the game presented is an RPG.

“Worldy”: an adjective applied to MMORPGs to describe RPGs with certain rather specific game characteristics, such as freedom of character construction, diverse rule systems including non-combat roles, etc.

“Social”: an adective applied to online virtual worlds to indicate that their primary emphasis is not on games.

Metaverse: the hypothetical idea that online virtual worlds would be linked together to form a single network.

Mirror world: an online virtual world that mirrors data from the real world.

You may join the debate already in progress.

FWIW, I think some of these numbers are wrong. But the overall point the list is trying to make is right.