A simple answer would be 'social proof'. Mashable provides a TopTen list of the most watched shows. That's a good start but the site is too distracting for Average Joe, in my opinion.
Also, web series are too short for true passive entertainment. I guess what your communities would need is a sort of Google Adsense, so you could plug-in advertising to be able to produce longer streams.
That would at least provide you with some money. When one show hits it big, that'll make news, and more people would start looking for shows like that. More eyeballs, more advertising, better chances of another hit, more news, and there you go.