I'm asking this in light of Facebook's IPO.

In comparison to computers, smart phones and tablets seem to be less friendly to advertising. I don't know whether this has an empirical basis, but to me screen estate is precious, and the same could be said for potential advertisers. I also noticed that a lot of the official mobile apps of established online services often lack advertising completely.

More importantly, unlike the internet connection at home, mobile data plans are usually limited. So you have a situation where the user ultimately ends up paying for the delivery of advertisements. How tenable is this?

How do companies like Google and Facebook plan to uphold their revenue streams in such an environment?