Begin from iOS6, Apple simplify the share process by using UIActivityViewController.
The thing is that, Twitter is only allow up to 140 characters on a post,
thus we may need to customize the share text for Twitter.

One of the way is to subclass the UIActivityItemProvider. Here is
how you can do it:

MyActivityItemProvider.h

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#import <UIKit/UIKit.h>

@interface MyActivityItemProvider : UIActivityItemProvider

@end

MyActivityItemProvider.m

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#import "MyActivityItemProvider.h"

@implementation MyActivityItemProvider

// 1.
- (id)activityViewController:(UIActivityViewController *)activityViewController itemForActivityType:(NSString *)activityType
{
    // 2.
    if ([activityType isEqualToString:UIActivityTypePostToFacebook]) {
        return @"Facebook: testing 123";
    } else if ([activityType isEqualToString:UIActivityTypePostToTwitter]) {
        return @"Twitter: testing 123";
    }
    
    return @"No provider";
}

@end

MyViewController.m

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// 3.
MyActivityItemProvider *message = [[MyActivityItemProvider alloc] init];

NSArray *activityItems;
if (myImage != nil) {
    activityItems = @[message, myImage];
} else {
    activityItems = @[message];
}

UIActivityViewController *activityController = [[UIActivityViewController alloc] initWithActivityItems:activityItems applicationActivities:nil];
[self presentViewController:activityController animated:YES completion:nil];
  1. Override activityViewController:itemForActivityType:.
  2. Customize message for each provider if you want to.
  3. Just treat it as a message, assign it together with image (if you have one). The UIActivityViewController will then pick the correct message for corresponding provider.