Leaving the merits of advertising itself aside for a bit, this post brings up a bigger topic that I think anyone running a startup needs to think about:
When you're getting involved in a startup, it is incredibly important that you be honest with yourselves and your partners about what the goal really is (this is to the point made the other day about self-awareness) and what you're really building.
A Feature: A piece of a piece of a puzzle. "It's like Facebook's News Feed, but instead you get audio updates instead of text updates!". Hackers can be guilty of this often - focused obsession on improving the world (or a piece of it) by tackling even the smallest problems. Not necessarily a bad thing, but not necessarily a guarantee of success.
A Product: Instagram is a product. It's full-featured, it's beloved (or was ;), it's free-standing, it works, and people use it. But do people pay for it? Would they? Would they make money by themselves or only in the context of a bigger company (like Facebook). Twitter is also a product, along these lines.
A Business: 37signals is a business. People pay them to use their products. They are free-standing, they are profitable, and they don't _need_ outside investors (need vs. want is different, btw - sometimes it makes sense to take strategic investment).
There's nothing inherently wrong with any of the above paths, btw. But you need to understand which path you're on, because you'll do a better job of it that way. If you decide to be on the feature/product path and hope to get acquired/acqui-hired, that's fine - it's a legitimate approach (that works, at least for now) and folks have made $billions doing it (see: Instagram & everyone else).
But it's a different path than starting a business. Goals are different, operations are different, and outcome is different.
The point, in the end, is that often the response - "well, I guess we'll sell ads" is what people who are starting features or products often say to convince themselves or others that they're on a worthwhile _business_ endeavor. And that's often dishonest (intentional or not).
Understand the path you're on and I'm willing to bet success will be more likely.