Here's how you can use Siri to search Twitter for tweets in iOS 7.
1. Press and hold the home button to start Siri.
2. Say "Search twitter for..." and Siri will begin a search.
3. If Siri finds tweets related to your search term, you'll see a list of them, with a link under them to see more on Twitter itself.
4. If you want some examples about what you can search for on Twitter, ask Siri this question:
"What can I ask you?"
5. Then tap on the Twitter icon to see a sample list of Twitter searches.
Siri can be a very fast and useful way to start finding tweets on Twitter.