You have to add account in your device setting,
Setting > Facebook
Setting > Twitter
Set it and than try,
NSURL *shareUrl = [NSURL URLWithString:@"http://www.captechconsulting.com"];
NSArray *activityItems = [NSArray arrayWithObjects: shareUrl, nil];
UIActivityViewController *activityViewController = [[UIActivityViewController alloc] initWithActivityItems:activityItems applicationActivities:nil];
activityViewController.modalTransitionStyle = UIModalTransitionStyleCoverVertical;
[self presentViewController:activityViewController animated:YES completion:nil];
If you share image also than related activity will display.