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?

Android Market advertising

When I submitted my first app 'Backgr' into the Android Market I was really proud of myself (I'm still too).
My first ever Android app born in less than a week (only 5 days of works), I needed to update it a bit (improve probably is the right word), but then finally was on the Market.
Then I decided to add advertising into my app, just because I had to pay an entry fee to Google and I want to be 'refunded' (in some way) by adv impressions.
Unfortunately, adMob doesn't support this feature. This means I need my users to click on the adv to earn some money. I think this is not the right way to earn money, because as a user (and not only a developer) I never click on in-app advertising.
adMob advertising is still into my app but it's almost useless right now, just because my total active users are only two: a friend of mine and myself.

My real problem right now rely on the lack of users, the need to spread my app around the world and I have to find the right way, without paying anything.

I think I'm going to ask some bloggers to help me in my 'mission'.
Let me know if somebody out there is interested