One major difference between Wikipedia and most online media is that it cites sources (and enforces citation as a rule, though enforcement is somewhat haphazard).

It is way easier to check some information if you are given an authoritative source for it. If Wikipedia tells you that some lizard men killed JFK, but cites no source for it, or cites some not obviously reliable source (say, a political blog), then just ignore that information. If Wikipedia says that according to some report, JFK was killed by such or such person, then Wikipedia will give you a precise citation or even Web link to the report.

So, in short, you're wrong. Sorting things on Wikipedia is easy if you simply bother to look for the citation links.

True, Wikipedia often catches the lazy, or those that lack the habit of reading footnotes and bibliographies.

AFP, AP, Reuters are not fail-proof. An agency (I think it was Reuters) once (mis)understood that Wikipedia was starting a search engine. This was a canard, and the Wikimedia Foundation, which hosts Wikipedia, issued a communique about it.

Nevertheless, the news got copied everywhere, and is still occasionally presented as fact by journalists.

This shows the vulnerability of modern journalism - a lot of it is basically copied from earlier articles, and a single error in a highly-placed source (say, AFP, AP, Reuters, or major newspapers) can be copied to many places without anybody bothering to check facts.