There is no "mainstream media" left in Russia after they closed Echo Moscow and TVRain (Doschd).
Propaganda actually works, that's why dictatorships keep using that playbook.
The point is that it's pointless to try to quantify how many fell victim to propaganda. There is no meaningful way to assess it, since all metrics you'll try to use are under control of a totalitarian state that controls every single statistic.
Mainstream doesn't mean a particular political affiliation or even objectivity. It just means the most widely referred to.
Dozhd and Echo Moskvy were never "mainstream", quite the opposite.