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.
> 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.)
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...
But ok, here's an experiment. Not snopes (I have snopes examples too, just requires a bit more time to find).
Bernie says black youth real unemployment is 51% - Politifact says it's mostly true[1], maybe even over that. Maybe the terminology was a bit off, but the point of high black youth unemployment is correct. Not completely true, but mostly is.
Trump says black youth unemployment is 59% - so Politifact says it's close to true, but probably closer to 51%, right? Trump is close to truth, but maybe exaggerated a little, as is his habit? Nope, Politifact says it's mostly false[2] and the real figure is a third of that. Note how in the first case PF is completely OK with extending unemployment definition but turns rigorously pedantic in the second case and insists unemployment is nothing but the figure published by BLS, and Trump's point - which is exactly the same point as Sanders had! - is total bullshit. How do you like them apples?
[1] http://www.politifact.com/truth-o-meter/statements/2015/jul/...
[2] http://www.politifact.com/virginia/statements/2016/jun/20/do...
Want more? Here's more.
Ron Paul says there was no income taxes until 1913? Half true[3], there were short-lived efforts to introduce the tax (as if that was the point). Jim Webb says the same? Fact check is now "mostly true"[4]. This one was corrected postfactum, because somebody noticed and made fuss out of it.
There are many more examples of PF doing this - spinning the facts depending on who said it. Snopes is doing the same, and more - like taking a factually true statement, finding some retelling that adds a slight exaggeration or twist to it and slapping "mostly false" on it and dedicating most of the article to refuting that slight twist and ignoring the original fact. Because while the initial fact is true, this particular exaggeration or twist is not, so the compound statement is "false" and now the original statement can be reported as "found to be mostly false by factcheckers". Primitive manipulation, but it works.
[3] http://www.politifact.com/texas/statements/2012/jan/31/ron-p...
[4] http://www.politifact.com/virginia/statements/2015/aug/24/ji...