gistfile1.m

- (RACSignal*)friendsId
{
    return [self enqueueWithCursor:-1];
}
 
- (RACSignal*)enqueueWithCursor:(NSInteger)cursor
{
    @weakify(self);
    return [[self idsAtCursor:cursor]
        // Map each `next` (there should only be one) to a new signal.
        flattenMap:^(id json) {
            @strongify(self);

            // Prepare a signal representing the next page of results.
            NSNumber* nextCursor = json[@"next_cursor"];
            RACSignal* nextRequest = [RACSignal empty];
            if (nextCursor && ![nextCursor isEqual:@0]) {
                nextRequest = [self enqueueWithCursor:[nextCursor integerValue]];
            }

            // Concatenate the results of this page with whatever comes from the
            // next page.
            return [[RACSignal return:json] concat:nextRequest];
        }];
}
     
- (RACSignal*)idsAtCursor:(NSInteger)cursor
{
    NSDictionary* parameters = @{ @"cursor" : [NSString stringWithFormat:@"%d",cursor] };
    NSURLRequest* request = [self requestWithMethod:@"GET" path:@"/ids.json" parameters:@{@"cursor" : @(cursor)}];

    @weakify(self);

    // Using this method instead of a subject ensures that we don't start the
    // request until someone subscribes to the result.
    RACSignal *requestSignal = [RACSignal createSignal:^(id<RACSubscriber> subscriber) {
        @strongify(self);

        AFHTTPRequestOperation* operation = [self HTTPRequestOperationWithRequest:request
                                                                          success:
                                             ^(AFHTTPRequestOperation *operation, id json)
                                             {
                                                 [subject sendNext:json];
                                                 [subject sendCompleted];
                                             }
                                                                          failure:
                                             ^(AFHTTPRequestOperation *operation, NSError *error)
                                             {
                                                 [subject sendError:error];
                                             }];
        [self enqueueHTTPRequestOperation:operation];
    }];

    // Kicks off this request only when subscribed to, and makes sure that
    // a RACReplaySubject is used to buffer values.
    return [requestSignal replayLazily];
}

I think you forgot to return a RACDisposable when creating the signal in idsAtCursor:

...
    [self enqueueHTTPRequestOperation:operation];
    return [RACDisposable disposableWithBlock:^{
            [operation cancel];
    }];
}];
// Kicks off this request only when subscribed to, and makes sure that
// a RACReplaySubject is used to buffer values.
return [requestSignal replayLazily];