Months since when, exactly? Most popular fact checkers existed long before the "fake news" meme brought them to newfound attention.
Since they gained widespread popularity as a watchdogs and arbiters over the "regular media". This was long before last US elections which has triggered the "fake news" panic (well, triggered may be not the best word here, "involved" may be better, but I don't want to digress too much).
Well, I realize it's hard for me to give specific times here, mostly because I don't remember neither when exactly I started reading fact-checking sites, nor when exactly I started noticing propaganda showing up. Maybe some of them held for a year or two, and maybe some of them worked in relative obscurity for a while and managed to keep a clean record before they gained popularity that allowed prominent politicians to cite them as a proof, and thus became a target for subversion. And maybe there are others who I didn't know about and who have much better record (bad luck for me).
The underlying point is that what I observed is that once some outlet becomes widely known as arbiter of truth in any political context, and it does not have very strong independent filtering mechanism, propaganda shows up very soon. If it does have one, propaganda shows up too (e.g. there are tons of biased articles on Wikipedia and there is a lot of shoddy and biased research in scientific journals) but a bit slower.
I've seen criticism of Snopes, for example - but apart from one or two examples of cock-ups that they corrected, I've never seen any evidence of subversion - so which sites are you talking about?
[0] http://www.snopes.com/hillary-clinton-freed-child-rapist-lau...
> Key line: "Obama took GM and Chrysler into bankruptcy and sold Chrysler to Italians who are going to build Jeeps in China." Romney's wording of this statement in a prior speech was even stronger: "I saw a story today, that one of the great manufacturers in this state, Jeep, now owned by the Italians, is thinking of moving all production to China."
> The claim originated in a Bloomberg News story, which the conservative Washington Examiner construed to mean that Jeep was struggling and considering moving some or all manufacturing from the U.S. to China as a result. When the Romney campaign was questioned about this statement, aides conspicuously stuck to it.
> In an important way it is the opposite of the truth. Between the 2009 bankruptcy filing and 2012, Jeep had robust growth. Its manufacturing in the U.S. was expanding and it was hiring workers. Its overseas sales also picked up, and it was considering re-opening shuttered factories in China. These facilities were not going to replace domestic manufacturing -- they were for making certain models of compact SUVs specifically tailored to the Chinese market, which are impractical to import from the U.S. So the statement, "Jeep is doing poorly and so will move manufacturing to China," is false, while "Jeep is doing well and may resume manufacturing in China," is true. (Furthermore, the deal that left Fiat with a controlling share of Chrysler emerged from talks begun before Obama was president.)
