A growing number of restaurants and fast-food chains use to Twitter to counter negative reviews and persuade skeptical diners to give them a chance. Eateries monitor Twitter for mentions, then try to lure customers with friendly conversation and discount vouchers. "It's time and resources very well spent. You can either pretend that [the conversation] isn't happening or decide not to be part of it. To us, it just really makes sense to use those as tools," says Chris Arnold, one of Chipotle's social-media managers.