How could they earn money?

Some hours ago I was thinking about the hundreds of free apps that Android Market offers.
Lots of them have advertising popping out from the layout.
But there are still a lot of them that don't even have a single adv link into the app.
So, I was wondering how could they really earn money from their work. Okay, probably lots of them are not looking for money in this exact moment, but only for popularity.
It's totally right thinking that popularity will lead them to a lot of money, but in the meanwhile what is happening?
Do they probably own a websites where displaying advertising is more money-efficient than adding in-app advertising?
Yeah, probably. Probably I will use the same concept in my app for some reasons as well:

1. I don't like advertising at all into mobile-app and I think they could slow down my app performance
2. In-app advertising let you earn money (poor money usually) only when user click on it and I know that most of my friends and colleagues would never click on an advertising even if it could seems really interesting
3. Advertising needs data connection and use it as well. This isn't so good, especially when you don't want to download more data than you need

Another important point is that advertisement needs space. When I work on a website or some web project I don't mind too much at the space. In the last years monitor are getting bigger and space is no more a problem. Right now, design principles start considering layout from a size of about 800 pixels (and sometimes even more). 

But when you have to think about mobile apps, the available space starts from 200px (very small indeed).

I have to find better way (and less time-expensive) way to earn money with mobile apps.

Do you know any of them?