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Some here think MMO is only about persistent world games. Well, obviously the industry disagree.
In particular, in this industry analytics firm, which publishes MMO market report as early as 2009. Their reports are used in many gaming site articles. They classify MMO into the following types:
and with settings like: Fantasy, Sci-fi, Realism, Anime, Horror, and History.
And they count LOL, and WOT (i guess in the battle arena category). This clearly shows the trend in the industry is expanding, and evolve into more variety. The old way of thinking about MMOs as games only like UO and EQ is a thing of the past.