Not saying that this problem never existed in the past, but it is far far worse now.
Go watch Walter Cronkite's reporting on the Kennedy assassination. He and his news room is just reporting the facts as they get them with no editorialization or agenda.
There's not a specific moment in time. There are specific innovations that accelerated and commodified the dissemination of facts, which each contributing to this decline in journalistic integrity and greater faithfulness to facts.
So is your gripe generally applies to all contemporary journalism or specifically with CNN and Fox News? Isn't it just as likely that legacy news sources are still doing the news pretty much the way they always have but that the availability of alternative sources has allowed people to diverge their opinion from a mainstream one more than they could before? (Which I'm not saying is good or bad. I think it likely has benefits and drawbacks.)