Like DSA said, Dark Shadows' ratings peaked in 1969 during the 1897 storyline. They began falling during the last half of 1897 and Leviathans, but still remained respectable. In the last half of 1970, the ratings dropped badly, which led to it's cancellation. Interestingly enough, the ratings began to go up during its last full month on the air.
I personally think the show never really lost its "touch." The show wasn't very good during the summer of 1968 (Adam...Dream Curse...*shivers*) and from November 1969 - May 1970 (Leviathans and first half of 1970PT...the first half of Leviathans wasn't too bad, and I liked the second half of 1970PT).
Other good periods of the show (other than the 1966-1968 pre-Adam episodes), IMO, were the haunting of Collinwood in late 1968/early 1969, 1897, the second half of 1970PT, 1840, and 1841PT.