How easy is it to create a facebook application?

Assuming you are familiar with one of the languages that there are Facebook API libraries (check out the Developers wiki for a list of them), it should be relatively straight forward.

There are a couple of strange things that are not similar to others that are due to the implementation of embedding your application inside Facebook:

• The place where you are developing your Facebook application must be accessible to the world. The way Facebook applications work is by posting their data into your application's page, so even for testing purposes your application must be accessible to the world (and of course to the Facebook servers ).
• Some of the actions in Facebook will not occur immediately like updating the user's profile box.
• Learning FBML might be a bit annoying at first but makes the experience for your users from Facebook a lot smoother.

All in all, it took me a couple of hours (under 5) to write the hCard application in PHP.

The application is very simple in Facebook API terms since it uses two APIs, one to get the user's information and the other to set the profile box information in the user's profile (setFBML).

It simply adds the hCard microformat support to your Facebook profile. 

To perform a stronger integration with a full blown application like Yedda Questions & Answers Facebook application () it takes a bit more time.

There are pros and cons for both (like everything else )..

Pros for FBML:

• Will look a part of Facebook (depending on how you use it but in general it will look like a part of it)
• Speed - IFRAME takes longer to load
• Perfromance - Facebook works in a way that it posts using an HTTP POST to your page, gets back FBML and renders it. You can use an FBML/IFRAME hybrid in which your FBML renders an IFRAME - it will look more native but will take at least 2 request on behalf of the client to render

Cons of FBML:

• Limited functionality - no JavaScript so no funcky things (or in a very very limited way
• You'll need to recreate your application's User Interface (if you already have one or adding a facebook app for an existing application)
• Performance - Your server MUST reply within a certain period (around a maximum of 10sec) so if you have something that is a bit more intensive Facebook might show an error page.

1)How to register application programmatically which API we have in that case?

2)How to add application in Facebook,what different API it requires?