This is the brilliance of the facebook platform. If someone comes up with an innovative idea and releases a facebook application that proves valuable to users, facebook can just build its own version to kill it and then build off its success. Its almost like free R&D.

Walled gardens are great.

Walled gardens are great, but walls keep people out as well as keeping people in.

http://www.joelonsoftware.com/articles/fog0000000052.html

Indeed. If I ever build a platform, the only circumstance I'd ever make someone else's work native to the platform is if I had their full permission, and even then I'd give them full credit.