I'm not a big Facebook fan. In fact, I don't use it long enough to form any opinion about it other than objection that it's a silo from which I can't get my data back out. But, since most of my users in library are using it, I decided to take a look how hard would it be to expose part of our library catalog on Facebook.
Facebook applications (in canvas mode which I'm using) are in fact simple web pages with a bit of custom Facebook markup. My initial gripe about applications was that they where slow. Now that I know that they are running somewhere else and not on Facebook I understand why they are slow.
So, to sum it up: if you know how to make simple CGI script, you will be fine with Facebook applications. They will be even slower than your application on your server, and if you do something popular you might have problems with server load.