Easiest hello world I've ever made was in Commodore 64 basic. The most difficult one was with Symbian -- I never had the patience to finish it! Facebook seems to fall much nearer to the C64 experience. Starting to develop a Facebook app feels really sweet. Seeing what you create instantly embedded on a major site is a strange feeling, because it's something that's completely new.
The thing I was most confused about at first was how the content you create would find its way to appear on a Facebook page. At first I figured that code must somehow be running on Facebook's servers, like in a sandbox. But it doesn't work like that, rather Facebook calls your server and displays the response it gets as part of the page it renders.
One surprise was that things are a lot simpler than you would expect. The PHP API include file is just around a thousand lines of code! I feel that now is the time to get in the game of building Facebook apps, because certainly after a while they will add a zillion new things you can do with the platform, which will make it even better, but also that much more confusing for those just starting out.
In my mind Facebook has won. With all the apps that have been created and are being created, Facebook will have thousands of times the functionality that MySpace is offering. The change in Facebook has been so sudden that people have not yet realized what happened, it will take a while before people understand that there are now these things called "apps" and that you can add them to your profile. People will gradually learn, and a few of those apps will be such gems that people will wonder how a social network could ever be considered complete without them.
