Google SideWiki, Brands in Public and many other pieces of software let anyone build content around your pages without your permission.
You shouldn't be afraid of people doing this, you should be afraid if people don't do it.
The barrier to creating communities for brands we love has totally collapsed. Anybody can build a community about your brand in a matter of minutes.
…so why aren’t they? Shouldn’t you be more disappointed that people aren’t grabbing these simple technologies and going crazy with it?
Seriously, why aren’t they?