What defines a celebrity? Showing up in the media and being well-known by the general public, as a cause or consequence of that. We also have local celebrities (the man who works in the butcher shop and is dating a singer), or family celebrities (like the uncle who was a poet and commited suicide when he was young). Whatever the case, being a celebrity means being famous, being part of popular culture in a certain scale.
Internet, in general, and YouTube, in particular, have already created their fair share of celebrities, some out of complete anonymity, such as the dancer of the evolution of dance; others fabricated, like LonelyGirl15, who gave way to a whole genre of scripted reality series. However, they are, at their core, celebrities like any other, just in a different environment.
Microcelebrities have a different status, though. We are talking about the microcelebrity, when a blog, Flickr or Youtube poster, who has created some services as an example of his genre wins readers and an audience beyond his inner circle of friends and acquaintances and starts drawing the attention of a small community of strangers on the Internet.
A few of the new platforms, Twitter, for instance, have invested in this visibility dynamic on a micro scale, precisely. Those who are popular here are not those covered in the media. It's whoever has the most friends and comments on their Hi5 or Orkut profiles. It's those who have more hits on YouTube or Flickr. And the definition of celebrity is bound to change as the concept of 'friend' and 'acquaintance' changes.
The truth is, from the moment you become a creator on the Internet, you yearn to become a microcelebrity, one way or another.