Number of On-Line Subscribers Tops 6 Million: On-line computer services saw their popularity boom last year. The number of subscribers jumped nearly 40% as competition pushed down prices, a recent survey shows. Many of the 1.7 million new subscribers signed up during the last three months of 1994 and the total number of users rose to more than 6.3 million, according to the Washington-based Information and Interactive Services Report. The newsletter attributed much of the increase to America Online Inc.'s addition of 500,000 customers since October, bringing its total to more than 1.5 million subscribers. America Online, CompuServe and Prodigy are the biggest of the 55 services surveyed; they have about 80% of the subscribers.