ONE meets all types on the Keeping Posted beat, and a certain well-heeled person we met up with at the Hong Kong Ballet ball left us rather confused with her words of greeting. ''It's so good to see you here - you must get invited to all the balls,'' she said, flattering us with her attention. But then she went on: ''I know you were also at the Philippe Charriol ball and the Friends of the Earth ball because you wrote about it. Your column is full of balls.'' Er . . . you mean as in banquets and balls, right, madam?