I first heard about the Echelon/5 eyes network on 60 Minutes some time in the early 90's. I remember controversy about it flagging people for mentioning "bomb" in phone calls, so I would say I did hear about it from the main stream media.
People still called it a crazy conspiracy theory when I brought it up, but I usually attributed that to them not watching enough news, not watching too much.
I'm still pretty sure the stories about flagging calls for mentioning keywords in the 90's were urban legends. The sort of computing power to do that didn't exist then.
It's a conspiracy theory pasted onto a kernel of truth (Echelon).
Even after snowden there is a lot of the same stuff going on. The stuff he leaked sure didn't show that the NSA was doing some sort of active analysis on all communications. Though at least now it's probably within the realm of technical possibility.
Modern institutions may or may not be trustworthy, but AFAIK they weren't laughing off "vast conspiracies" which later turned out to be true. For example, I'm not aware of any mainstream media which laughed about the echelon program which later turned out to be true.
It's true that mainstream media didn't write about these theories, but one can easily argue that was reasonable given the lack of evidence.
It's important to note that details matter when reporting news, and these details were unknown prior to Snowden. It's also true that often the details get ignored (as one can see by the number of people who think that the PRISM program says that Google/Apple conspired with the NSA when in-fact it says the opposite), but I'm not sure that is completely the media's fault.