Trust is valued in communities, including internet commmunities, and people have observed this since the late 90s.  The newer social media properties are no different.  Everyone knows there are fakes on the internet.  I mean, duh.

I guess Lisa Barone's saying, and I agree, that the intent and action of the avatar matters, too.  Ronald McDonald isn't real, but that's OK because everyone knows he represents McDonalds.  Some normal-seeming person who really loves McDonald's hamburgers might be OK, too, but if that person were paid by McDonald's to hype their product, that would be deceptive and upsetting to the community.

It's like when journalists disclose monetary relationships with their subjects: if they don't disclose any money ties and the relationship comes out, they risk losing credibility.

What about the fake persona who doesn't hype a product, or who even disses it?

A couple years ago I posted on the searchenginewatch.com forums under the name "hutcheson" and pretended to be an Open Directory editor lamenting the decline of that directory.  Now there is an ODP editor who uses the handle hutcheson and who was vocal on messageboards (although not that one.)  I hoped that people would think my post was from him.  My action was a prank, a bit of culture-jamming.  In some sense my action was like that of "Katie Cake" in Lisa's example because I intentionally deceived people.  But in another sense, I wasn't trying to sell anything, and the readers of that forum were surely savvy enough to doubt messageboard posts.

My post did, however, undermine trust (and it was rightly removed) - that's what pranks do.  I don't think anyone who has spent more than a few months on the internet is surprised that these things happen, though.  The web is full of fakes.  The real world is full of fakes.  Check out the Orson Welles movie "F for Fake" sometime.