I think there are a few tactics in play.
First of all, I think it appeals to a sense that many people have that the media is not always reporting on important things. And certainly in modern times, with all these allegations of 'fake news', many people look at the media as untrustworthy and pushing certain agenda's. Now, whether this is really the case is not the point .. the point is that many people believe this is true, and that's what it appeals to.
Of course, one logical problem with this line of 'reasoning' is the following: even if it is true that the media does not always report on important news (i.e. there is some important news that the media does not report on), does not mean that whatever the media does not report on is important. For example, the media is not reporting on the color of the socks I am wearing today ... but that does not make it important. And the media is also not reporting on me swimming across the Pacific Ocean this morning ... but that does not make it true.
And yet, I think many people are making this fallacious logical inference:
"It's not reported on by those untrustworthy and biased media. Ah! So then it must be important and true! I better FWD to grandma!"
Not sure what technical name to give to this tactic. But if the tactic points out a specific group that is not reporting on something, then the argument involves a clear Circumstantial Ad Hominem, combined with a bit of Denying the Antecedent (if reported by group G, then false ... so if not reported by G, then true)
Second, I think the tactic is appealing to a sense of duty: "this is important ... and the media isn't reporting on this .. so it's your duty to spread the word!" So here maybe it's not so much the media being untrustworthy, but rather that the media may simply have 'missed' this very important thing! And so yes, do your duty and help spread the word!
And, of course, everyone wants to feel like you're doing something important and help out, and be a good citizen and patriot and all.
In the end, though, it is once again purely emotional and rhetorical: by making reference to the media, the suggestion is that we're dealing that should be reported on (unlike, again, the color of my socks); hence the 'duty' we have to FWD to grandma.
And is it important? Well, that remains to be seen ... "Bram is wearing his blue socks today! And the media isn't even covering this!! Do your duty and FWD to grandma!"
So, I would classify this as a fallacious (emotional) Appeal to Duty