In 1972, a survey conducted by the Gallup Poll asked "how much trust and confidence do you have in the mass media--such as newspapers, TV and radio--when it comes to reporting the news fully, accurately, and fairly--a great deal, a fair amount, not very much, or none at all?" The question was asked again in 1974 and 1976. Then there was a long gap until it was asked again in 1997, but since that time it has been asked pretty regularly. The means, with a great deal=4 .... none at all=1:
There were a few surveys which asked about the "news media" rather than the "mass media": they are shown in red.
In December 2016, I wrote about other Gallup questions about confidence in various institutions, include newspapers and TV news. Both showed a downward trend, so it's not surprising that confidence in the "mass media" does too. However, the question that I just discovered helps to shed light on the nature of the trend. The following graph shows them all together (the mean is adjusted so it'a on a comparable scale):
With newspapers, there is an unusually high figure in one year (1979). If you exclude that, there is very little trend from the 1970s until the early 2000s. Similarly, TV has one unusually high year, which happens to be the first year it was asked, and then no trend until to the early 2000s. When you add the question on the media, it's pretty clear that there was a decline from the 1970s to the 1990s, but that confidence then held up for several years before starting to decline again.
Another interesting point is that confidence in the media rose from 2016 to 2017 (September in both years). This also happened with confidence in newspapers and TV news. An obvious possibility is that the gain was a result of reporting on Donald Trump. That might have pleased some liberals who in 2016 thought that the media was too hard on Hillary Clinton and/or Bernie Sanders. I think there may also be a general tendency for it to be lower in election years: it rose between 2012 and 2013, 2008 and 2009, and 2004 and 2005 (it was the same in 2000 and 2001). That could be because people get tired of "horse race" coverage.
[Data from the Roper Center for Public Opinion Research]