This question is about sites that host community-contributed content, like Wikipedia, Reddit, or blogging platforms like LiveJournal communities. Suppose somebody posts something negative about an individual on such a site, something that might or might not be lashon hara (it's not clear). What does halacha say the overseers of the site (Wikipedia admins, etc) need to do in response if they should happen to be Jewish? Consider several cases:
The negative content is already widely-known (been in the press, for example).
The negative content is not already widely-known or is even new.
The negative content wasn't directly posted to your site but was linked.
The negative content has been removed from the "main view" (Wikipedia article, blog front page, etc) but is still available via a specific URL.
How far does a Jew who happens to be in an administrative role have to go to prevent the spread of possible lashon hara in cases like this? Is it enough that the information isn't directly on his site, or is there but buried, or does halacha call for him to purge it if that capability is available to him?