I guess this paper is a “meta” paper, since it discusses the requirements for extraterrestrial intelligence as well as one of the first SETI experiments.
The paper directly addresses the various problems surrounding the likeliness of us discovering intelligent life, in particular the mystery (now mostly solved) behind planetary formation, the mystery (slightly explained by Miller’s experiment) behind the formation of life, the timescale of life, and the timescale of intelligent, communicative life. The majority of the paper is a sort of thought experiment, where Drake goes point by point through his equation (not mentioned) and delves into specific factors effecting each of the compounding probabilities; he plays around with numbers that seem (to someone fairly unfamiliar with SETI) sound. However, as Drake points out, there is no way for us to be sure if he is correct or not unless and until we actually find life. I personally am glad he included this, since I feel with such statements, some might misconstrue his theory to be real and supported. The media nowadays would go nuts (perhaps they did then as well).
Drake also introduces Project Ozma, the first search for intelligent communications via the National Radio Astronomy Observatory. In this search, they followed up the suggestion by Coccini and Morrison 1959 and studied two of the closest solar-like stars for radio emission. Drake agreed with both Coccini and Morrison 1959 and Bracewell 1960 that a logical (possible the most logical) frequency to study would be that of the neutral hydrogen 21cm line, since civilizations early in their astronomical technologies would start here. These two stars, Tau Ceti and Epsilon Eridani, were studied for a total of 150 hours. No evidence for intelligent communications was found, nor was it really expected apparently. As far as I know, many searches similar to this have been made over the years, and I don’t think any have led to direct evidence.