This is assuming all reach is equal. Number of outlets seems like an incomplete picture to me where the number of people reached seems more important. Is a more concentrated effort by one group somehow less insidious? I think not.
But to your point, we don't know unless we know, and my assumption could be totally wrong.
1) Pareto: where there's a power law based distribution. So it is possible that 5% of sites have more than 50% of impact.
This just means it isn't out of the bounds of possibilities. But we'd have to dig deeper into which sites garner the majority of eyes (though we'd expect a power law in this).
2) Left learning areas tend to be more population dense.
Of course with these two considerations we haven't really made any conclusions. I just want to point out that a small number of sites can reasonably hold the majority of viewership. Which sites those are and which bias they entail, that requires a real analysis. We've just stated that the conjecture isn't impossible.