Bill Maher endorsed Sanders a day or two ago (I think, I'm not really sure of the exact day or when his show is on - maybe on Fridays?). We all know that he donated no less than a million dollars to Obama's campaign in 2012. I'd personally say that he has the second biggest bully pulpit in all of the US, after Obama. So this could make a difference, could it not? I think Sanders is pretty flattered by this endorsement. Much more so than he was by the endorsement by Seth MacFarlane and other celebrities for instance, lol. Maher endorsing Sanders should mean a lot for him for reaching and getting to the polls the youth vote in primaries and caucuses, and especially so in more liberal states.
I think you're overestimating the reach of these guys.