CNN Ratings Go South. Calling O.J., Calling O.J.
Article discusses how Cable News Network ratings have fallen in last year; despite intense coverage of Senate hearings on campaign finance abuses, CNN drew only a 0.4 rating, which represents viewers in fewer than 300,000 homes; network is receiving even lower ratings for its coverage of Pathfinder mission to Mars; network's average Nielsen rating in second quarter of 1997 was 0.39, lowest 24-hour rating in CNN's 17-year history; graph (M)