I am beginning to write a Facebook application. I have read many articles about the subject and have learned that Facebook has an official PHP API, and .NET has a Facebook toolkit from Codeplex.
Both options have many references and resources, but it seems that Facebook itself was written in PHP, not .NET.
So does PHP or .NET have better integration with Facebook?
Well its depends on what you are trying to accomplish. Do you want to create a little application that checks for updates on your profile i think you should use the API for that.
But it all depends on what your goal is
facebook employs a RESTful api (http://wiki.developers.facebook.com/index.php/API). It doesn't matter where the requests are coming from, as long as they are well formed according to their specs, it can come from C#, C++, vb, php, asp.net, it simply does not matter. Use whatever suits you best.
It sounds like you would rather do it in .NET then in PHP, so here's what I think:
Facebook is in PHP (at least on the front-end), so the app may integrate smoother if you write in PHP,
However, since you prefer .NET, I'm guessing you are more confident with it, so you are less likely to make mistakes, will be able to debug faster, and will be more creative,
Do you have any .NET examples you can look over? If all the .NET apps are sketchy, crappy, awkward, or pushy, that should give you a pretty good idea on .NET's track record for Facebook.
So unless you find a solid example/reason for not going with .NET, I'd go with whichever you prefer.
On the other hand, maybe you are asking because deep down you know that PHP is awesome and it's time you got comfortable with it. If that's the case, now is as good time as any to take the plunge.