Making Facebook apps costs, complexity etc?

Hi all,

I’m about to start building a new website and am just having some thoughts of where I would like it to be in 6 months.

One thing on the wishlist would be to get some sort of integration with Facebook, probably my own Facebook app, where everytime the user does something on my site their friends get told about it.

I’m thinking of how Farmville tells everyone you know everytime you do something, such as level up etc. It’s constant free advertising basically.

Anybody have any experience with making Facebook apps? Who can make them? Any reputable companies out there with keen prices?