This problem will occur on iOS6 as well as iOS7, since /1/statuses/update.json
is now deprecated. Also, beginning with v1.1 of the API, you must use HTTPS for all requests.
Your URL should be changed to the following:
NSURL *url = [NSURL URLWithString:
@"https://api.twitter.com/1.1/statuses/update.json"];
For more information, please see Twitter API FAQ: What does the retirement of API v1 entail?