Do you plan on making actual business of it? Then I'd say – probably not.
The CPM of Google AdSense, and the likes, are usually about 10-25 cents CPM. For bloggers and other low-cost online sites that should cover the basic costs, and even make some money if you're big. To make actual money on a site, you need more than that.
Andrew Chen have written well about how "Your ad-supported Web 2.0 site is actually a B2B enterprise in disguise", which is worth the excellent read. His point being, to make actual money you're going to have to sell your ad space yourself, by providing a good audience segmentation (what's your visitor demographics) to a suitable advertiser.
But as with everything, there is another side to this story.
It's a start. And perhaps an important start. Maybe it covers your server costs, etc, allowing your a few extra hours a week to produce new features, or grow your audience? AdSense may very well be on the path to your sustainable business model.
However, it's important that your bet isn't "I have to grow huge before I can grow profitable". Very few businesses succeed at that, and the costs are immense. Your investors know from the bubble days that it's not a bet worth taking. (Even Facebook, at > 500M users isn't really profitable)
In short, day one you need to have an idea what your business model will be. And day two, you start exploring if the idea you have can generate any sales ("maybe these guys would pay for promoted posts", "maybe if I charge for this extra feature", etc.) If it works, try scaling it, if it doesn't throw it away, come up with something new and try that. Iterate quickly.
Wow, I feel like I can go on forever. But my basic advice is: have an idea and start exploring it early.
