Her eponymous 8pm show's viewership has trended down steadily, and she just had her lowest average viewership night ever on Tuesday (2/23) with just 332,000 average viewers. She's averaged 537,000 daily viewers in February through 2/21/10. When asked, CNN pointed out that the Olympic coverage always affects their ratings.
Here are some charts of her long term viewership declines.
First, a full chart.
And now one with the 2 big peaks cut off for more detail on the rest of the days.
Update: I edited both charts to clarify that they contained average daily viewership, corrected Campbell spelling and clarified that the vertical axis numbers were thousands of viewers.