Facebook access token error on iphone - iphone

Question

I am trying to get name of facebook user in my app but I am getting
{"error":{"message":"An active access token must be used to query information about the current user.","type":"OAuthException","code":2500}}
My code is :-
In viewdidload
facebook = [[Facebook alloc] initWithAppId:#"233350100117670" andDelegate:self];
NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
if ([defaults objectForKey:#"FBAccessTokenKey"] && [defaults objectForKey:#"FBExpirationDateKey"])
{
facebook.accessToken = [defaults objectForKey:#"FBAccessTokenKey"];
facebook.expirationDate = [defaults objectForKey:#"FBExpirationDateKey"];
}
NSArray *permissions = [[NSArray arrayWithObjects:#"read_stream", #"publish_stream", #"offline_access",nil] retain];
[facebook authorize:permissions];
if (![facebook isSessionValid])
{
[facebook authorize:nil];
}
After that:-
- (void)request:(FBRequest *)request didFailWithError:(NSError *)error
{
NSLog(#"%#", [error localizedDescription]);
NSLog(#"Err details: %#", [error description]);
}
- (void)request:(FBRequest*)request didLoadRawResponse:(NSData*)data
{
NSString *response = [[NSString alloc] initWithData: data encoding: NSUTF8StringEncoding];
NSLog(#"%#", response);
[response release];
}
- (void) request:(FBRequest*)request didLoad:(id)result
{
if ([result isKindOfClass:[NSDictionary class]])
{
NSString *email = [result objectForKey: #"email"];
fbUserName = [result objectForKey: #"name"];
NSString *facebookId = [result objectForKey: #"id"];
NSString* title=[NSString stringWithFormat:#"%# has won a gold medal on Olympedia quiz",fbUserName];
NSMutableDictionary *params =
[NSMutableDictionary dictionaryWithObjectsAndKeys:title, #"name",#"Feed Dialogs of Olympedia.", #"caption",#"Check out Olmpedia Dialogs.", #"description",#"http://www.facebook.com/olympedia", #"link",#"http://bit.ly/PdpRSD", #"picture",nil];
[facebook dialog:#"feed" andParams:params andDelegate:self];
}
}
And on share button click:-
- (IBAction)shareOnFacebook:(id)sender
{
NSLog(#"Posting message");
[facebook requestWithGraphPath:#"me" andDelegate:self];
}
Please help as I am trying to solve this for hours.strong text