Some viewers are more valuable to advertisers (and hence a TV network) than others (source):
Conventional advertising logic is that the number of viewers of a television program within the 18-49 age range is more important than the total number of viewers. Television advertising rates are still, by and large, based on this premise.
Revenge is popular with the 18-49 age range, so this is probably the reason for it being renewed.
Other possibly reasons for a show with low viewership being renewed are strong DVD sales (e.g. Peep Show) or being popular overseas (Revenge is broadcast in over 30 countries).