Nike's market share, which peaked at 47.5 in 1997, has dropped a couple of percentage points every year since. In the company's latest earnings report, which showed a strong overall profit, footwear sales in the United States were down 13 percent. Competitors have a long way to go, but they have momentum. And they will be helped along by Nike's recent spat with Foot Locker, which by itself accounts for 11 percent of Nike's revenue. Foot Locker, sensing a growing preference among kids for other brands, decided to reduce the number of Nike models in its stores. In what John Horan, publisher of Sporting Goods Intelligence, characterizes as a ''scorched-earth, emotional response,'' Nike cut the chain off from its ''statement'' products, the hot new models with big promotional budgets.