I was looking for something similar myself, but there was nothing like this! All I found was either abandoned buggy open-source initiatives - or big, complex, heavy authentication libraries working with OpenID/Twitter/Facebook/blackjack-and-hookers...
So I dived into the Facebook's developer docs (the ugliest docs I've ever read BTW) and I created this very simple project that you can extend. Just two files, 10 lines of code each.
The only "library" I use here is the JSON serializer (to make the whole thing work under ASP.NET 2.0)
No download, just get the latest source code, the solution is very small, 3 files...
Read more: Codeplex