At kenHub - we build a platform for learning anatomy. It makes students' lives easier and they can be more effective in their studies. As much as we'd like to give it away for free to all students, we need to make a living, and we need money to improve our products. Luckily, our audience seems to see the value and are willing to pay for it.
I think it boils down to the utility of your product, to students, grandmothers, or any other segment.
Even when Facebook started, it was hemorrhaging debt in the beginning...and eventually, they went to ads (even with the outcry). User's usually hate change, each product is different, so try to constantly ruminate a method or revenue (maybe even for select users in your demographic).