Model class for networking that uses notifications

I just finished a model class that represents an Instagram Networking client for my iOS app.

I am specifically looking for a review of my use of static constants. These are strings that represent notification names and notification userInfo dictionary keys.

Here is my networking class:

POPInstagramNetworkingClient.h:

#import "AFHTTPSessionManager.h"
@interface POPInstagramNetworkingClient : AFHTTPSessionManager

+ (id)sharedPOPInstagramNetworkingClient;

- (instancetype)initWithBaseURL:(NSURL *)url;
- (void)requestPopularMedia;
- (void)requestMediaWithTag:(NSString *)tag;

@end

POPInstagramNetworkingClient.m:

#import "POPInstagramNetworkingClient.h"
#import <AFNetworking.h>

@implementation POPInstagramNetworkingClient

#pragma mark - Singleton
+ (instancetype)sharedPOPInstagramNetworkingClient
{
    static POPInstagramNetworkingClient *sharedPOPInstagramNetworkingClient = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{

        //Define base URL string
        static NSString * const BaseURLString = @"https://api.instagram.com/v1/";

        //Create our shared networking client for Instagram with base URL
        sharedPOPInstagramNetworkingClient = [[POPInstagramNetworkingClient alloc]initWithBaseURL:[NSURL URLWithString:BaseURLString]];
    });

    return sharedPOPInstagramNetworkingClient;
}

#pragma mark - Initializer Methods
- (instancetype)initWithBaseURL:(NSURL *)url
{
    self = [super initWithBaseURL:url];

    if (self) {
        //Set the serializers
        self.requestSerializer = [AFJSONRequestSerializer serializer];
        self.responseSerializer = [AFJSONResponseSerializer serializer];
    }

    return self;
}

#pragma mark - Instance Methods
- (void)requestPopularMedia
{
    //Define notification names
    static NSString * const kRequestForPopularMediaSuccessful = @"RequestForPopularMediaSuccessful";
    static NSString * const kRequestForPopularMediaUnsuccessful = @"RequestForPopularMediaUnsuccessful";

    //Create manager and execute GET method to retreive popular media
    AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
    [manager GET:[NSString stringWithFormat:@"%@media/popular?client_id=76566d0e6d5a41069ea5e8c86fbbd509", self.baseURL] parameters:nil success:^(AFHTTPRequestOperation *operation, id responseObject) {

        //Post success notification
        [[NSNotificationCenter defaultCenter]postNotificationName:kRequestForPopularMediaSuccessful object:nil userInfo:@{@"requestForPopularMediaResults": responseObject}];

    } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
        NSLog(@"Error: %@", error);

        //Post failure notificaton
        [[NSNotificationCenter defaultCenter]postNotificationName:kRequestForPopularMediaUnsuccessful object:nil userInfo:@{@"requestForPopularMediaResults": error}];

    }];
}

- (void)requestMediaWithTag:(NSString *)tag
{
    NSLog(@"requesting media with tag");

    //Define notification names
    static NSString * const kRequestForMediaWithTagSuccessful = @"RequestForMediaWithTagSuccessful";
    static NSString * const kRequestForMediaWithTagUnsuccessful = @"RequestForMediaWithTagUnsuccessful";

    //new - @"%@tags/%@/media/recent?client_id=76566d0e6d5a41069ea5e8c86fbbd509"
    //old - @"%@tags/search?q=%@&client_id=76566d0e6d5a41069ea5e8c86fbbd509"

    //Create manager and execute GET method to retreive media with tag
    AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
    [manager GET:[NSString stringWithFormat:@"%@tags/%@/media/recent?client_id=76566d0e6d5a41069ea5e8c86fbbd509", self.baseURL, tag] parameters:nil success:^(AFHTTPRequestOperation *operation, id responseObject) {

        NSLog(@"Success!");
        NSLog(@"Response Object: %@", responseObject);

        //Post success notification
        [[NSNotificationCenter defaultCenter]postNotificationName:kRequestForMediaWithTagSuccessful object:nil userInfo:@{@"requestForTaggedMediaResults": responseObject}];

    } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
        NSLog(@"Error: %@", error);

        //Post failure notification
        [[NSNotificationCenter defaultCenter]postNotificationName:kRequestForMediaWithTagUnsuccessful object:nil userInfo:@{@"requestForTaggedMediaResults": error}];
    }];
}

@end

I was told previously that when defining static variables I should keep their scope as limited as possible, so you will notice that I defined these inside of specific methods instead of at the top of this file.

One thing that is bothering me though is the fact that I have other classes observing for these notifications and also accessing the passed userInfo dictionary when the notification posts. I basically just copy/pasted the notification name's static string and userInfo dictionary key string.

This is definitely wrong, and I feel like these variables might need to be placed into a single file that would be included in the precompiled header, but I'm not sure if this is the correct approach so any advice on this would be greatly appreciated.

I want to make sure I am following best practices.