Procter & Gamble Co. is launching a marketing campaign that involves bars, singles and mobile phones. The promotion, for Crest Whitening Plus Scope Extreme Toothpaste, involves consumers in a text-messaging game through London-based Flytxt Inc., a firm that specializes in mobile marketing. It is the first in several campaigns the companies plan to launch, this one being in partnership with author and relationship expert Samantha Daniels. Here’s how it works: Procter and Flytxt will...