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1.
One who gains a cult or mainstream following due to viral internet distribution. Does not refer to those who have gained limited or cult followings through traditional media. Does not refer to has-beens or "B-list" celebrities.
Lonelygirl15 was nothing until YouTube made her a microcelebrity. Clint Howard is a D-list celebirty, not a microcelebrity.
by Brandon Li September 14, 2006
 
2.
A celebrity (Usually an actor/actress) who has not gained enough popularity to have their own Wikipedia page, but can still be considered a celebrity.
Examples of a micro-celebrity is someone who has had a few semi-major roles in a TV show/movie, but hasn't done enough such that they can be recognized by many.
by j0s June 23, 2014