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.