Procter & Gamble’s new Tremor unit is designed to target teens and have them spread the word about new products. “If we do it right, teens will spread the word naturally,” said Steve Knox, business development, Tremor. “It’s in their DNA.” P&G estimates teens spend more than $100 a week, or $172 billion a year. Executives said the new program is based on chosen teen’s internet usage, social networks and willingness to advocate to peers....