With information readily available, as stated above, people have become lazy in their research, and grasp at what is most readily available to them (for example, the first result in a Google search) regardless of how true it is, because it's the most popular perspective. To make matters worse, many don't care how much truth their is to their discovery, because their new truth is in line with what is popular.
I wonder if the perception of a decline in 'truth' is really just a decline in accepted orthodoxy (which is independent of truth) and a broadening of the field of public discourse.
Theres 2 important things:
1. In the past you had a much narrower range of public narratives, so less to disagree about and find fault with
2. A far higher orthodoxy regarding these narratives creating a perception of truth
In the past, more people believed the same things, and what most people agreed on was 'truth'. I'd say people are far less lazy with their research these days. In the past research was difficult and time consuming so few people bothered, they just consumed mass media. Anything outside mass media didn't really happen for most people.
While this was less partisan in the past, and also better funded with less reliance on PR sources of information, the mass media has never been a particularly accurate purveyor of information. People were far more trusting of 'authority' in the past though, so would take it at face value.
Just my thoughts anyway.