some Troubleshooting for your issue:
You probably already have your ClientID correctly set up. Check it
again on https://developers.facebook.com/apps
It took me a while to figure out what to enter as for the
RedirectURL. At the moment i am using
https://apps.facebook.com/yourappname/. If this is not working for you, go to your App on Facebook Developers Page > Settings > Add Platform > Facebook Canvas > And enter this url as for the "Canvas Page". Authentication should work fine now, and the 403 error should no longer occur.
My working example, for creating the Facebook service:
public static FacebookService Facebook
{
get
{
if (mFacebook == null)
{
mFacebook = new FacebookService() {
ClientId = "<Your App ID from https://developers.facebook.com/apps>",
RedirectUrl = new Uri ("https://apps.facebook.com/yourappname/")
};
}
return mFacebook;
}
}
For which Platform are you developing?
The rest of the share process is just like in the Xamarin.Social IOS Unified sample.
Don‘t hesitate to ask for clarification.