Three years ago, SKY DAYTON owned a pair of hip Los Angeles coffeehouses. Then he started EarthLink Network, an Internet service provider. On course toward having 600,000 customers by the end of 1998, EarthLink is fast creeping up on the Microsoft Network. But Dayton, 26, is eyeing the No.2 slot behind America Online. Revenues, which were just $3 million in 1995, were nearly $80 million last year. At 23, the stock was up about 75% for the year.