problem signing in with facebook

martin2008

was building a forum and added the api key and secret for facebook into it but i get this error. need help:
{
"error": {
"message": "Invalid redirect_uri: Given URL is not allowed by the Application configuration.",
"type": "OAuthException",
"code": 191
}
}

peregrine

http://stackoverflow.com/questions/8100043/facebook-invalid-redirect-uri-but-the-url-looks-fine-to-me

http://stackoverflow.com/questions/16438579/invalid-redirect-uri-given-url-is-not-allowed-by-the-application-configuration

http://salesforce.stackexchange.com/questions/32845/facebook-api-error-191-while-using-facebook-toolkit-in-salesforce