You mean "acronym", not "abbreviation".
Strictly speaking, it's neither; it's actually an initialism. Acronyms are pronounceable, like NASA and laser.
MeatFarts, the use of "a" or "an" is totally dependent on pronunciation, not spelling. MMOG is an initialism that's pronounced "Em em oh gee," so "an" is the correct definite article. If the author wanted to write out "Massively multiplayer online game," he would, and he'd use "a" to precede it.This comment was edited on Sep 17, 20:27.