Facebook applications are no different to websites
We've built plenty of Facebook applications.
One application we built a few years back was recommended by Facebook (back when they used to do at) and ranked in the top 5 most popular applications for a few months. No kidding, Obama used it.
We built a really cool app for Channel 10 to promote one of their shows: it really did go viral and they had tens and tens of thousands of entries.
One recent app - Winkiwoo - has gone global with sales in over 200 countries.
All good stuff.
What may or may not surprise you about these apps and others however, is the they are fact effectively standalone websites.
Facebook doesn't host them, you do.
Facebook doesn't give you any templates or code... You need to do it all yourself.
Except for being registered with Facebook and talking backwards and forwards, Facebook apps are quite literally websites in themselves, written in whatever technology you want.
Which is great on one hand, though a bummer if you should they were a hop, skip and jump to develop... Just like a website, they take time and money.
As with a website, you need to document what it will do and how data and fun tonality will be treated. As with a website, you need to do wireframes and a prototype.
You need to design the Facebook application and of course build the application.
In face, quite arguably, building a Facebook application is a little harder than building a website because where with a website you own your own destiny, with a Facebook application, you rely on Facebook for user data and accessing photos and so forth.
And much like a website, you'll still need the tools through which to manage your Facebook application and it's data. A content management system as it were.
Facebook applications - if well designed and promoted - can be very cool. You can collect a lot of data and drive a lot of participation if you're smart about it.
Don't however think they're simply widgets already sitting on the shelf of your web developer because they're not.
