As much as the RIAA's legal department might like to take credit for the decrease in song swapping, a couple of other factors likely contributed to the drop. For starters, the introduction of Apple's iTunes Music Store bought additional exposure for the various legal music download services currently available. Seasonal weather may also have had an impact on song swapping, and so may the fact that many students don't have access to campus bandwidth during the summer.
The RIAA's legal threats probably had a hand in at least some of the song swapping household decrease, but the number of songs downloaded per household actually went up from 59 to 63 between April and June. Overall, that still results in a net decrease in song swapping, but it will be interesting to see what happens in the fall when colder weather starts keeping people inside and students plug into campus broadband.