I wish more people understood this: if ads are working on your site it is because someone has figured out how to monetize your users. (At a multiple of how much you get from the ads!) Why isn't that someone you?

Conversely: if you are not charging money because your users are just piss-poor at actually paying for things (like most apps focused on music -- where most of the cool crowd hums "yar har diddlee dee, being a pirate is alright to be, do what you want 'cause a pirate is free, you are a pirate" under their breath), then ads aren't going to succeed either, because no one wants to buy ads targeting people who don't spend money. (Well, OK, Madison Avenue loves selling those ads and chalking it up to "branding", but advertisers are gradually learning to actually measure ROI on those spends, too, and they're terrible.)