Via Pat, a guy at the OC Register actually counted them, or at least estimated, based on stills from the Nats' TV broadcast, and came up with an attendance figure of about 1,100.
Obviously, the weather stunk, it was Monday, it was a make-up game, and the Penguins had a playoff game that night.
What does it mean? The Pirates' attendance figures are never among the league's best or anything, but beyond that, probably nothing. The Pirates' attendance actually increased last year even as attendance was down slightly throughout baseball despite the opening of a new ballpark in Minneapolis.
You can just chalk the weak attendance figures on Monday up to all the extenuating circumstances. I went to the game on Saturday, and the attendance was really nothing like Monday's game, even though there was a rain delay of over an hour. Also, the Pirates' attendance so far this year hasn't been atypical for April. I don't think the poor attendance on Monday reflects any change in fans' enthusiasm about the Pirates.