Traction.
It is believed that, if you can get a large amount of people engaged (ie. "traction"), there will be ways to make money of that. Sometimes it turns out to be true (Craigslist), sometimes not so much (MySpace).
It is also believed that there are multiple profitable markets being created right now on the internet (coupons, q&a sites, casual games, ...) and that this will continue for years to come. Even though we don't know exactly how the winning companies will make money in these markets, if you're the winner, it is thought that you have a good shot at figuring that out.
If you are fast growing in a promising market, you have a chance to become the winner of that market. (It is also believed that many markets will have only a few winners, due to network effects (Facebook), economies of scale (Amazon) etc..) And then the investors want in.
Mostly, investors don't invest in ideas. They invest in markets and teams. Teams can prove their worth by showing a proof of concept, or better, traction.
In Dailybooth's case: traffic and engagement.
There's no calculation where X traffic = Y $, because it's not linear. Growth is promise.