Is there a specific reason why Senator Thune is going after Facebook for this alleged bias? Why does there seem to be a separate standard of bias for Facebook v

Is there a specific reason why Senator Thune is going after Facebook for this alleged bias? Why does there seem to be a separate standard of bias for Facebook vs. CNN/MSNBC/Fox/etc? I'm not making a point with these questions, I'm genuinely curious if there is something else going on under the hood that I'm unaware of.