[National Research Council, "Informing Americas Policy On Illegal Drugs" (Washington, DC: National Academy Press, 2001): "It is widely thought that nonresponse and inaccurate response may cause surveys such as the NHSDA and MTF to underestimate the prevalence of drug use in the surveyed populations (Caspar, 1992)." (p. 93)]
Federal surveys show an increase in use, especially by adolescents, since 1990.
["Monitoring The Future: National Survey Results on Drug Use, 1975-2000, Volume 1: Secondary School Students" (Washington, DC: National Institute on Drug Abuse, 2001), p. 115, and and "Volume II: College Students and Young Adults Ages 19-40," p. 102.]
More reliable data on trends is overdose deaths and emergency room mentions of drugs. These numbers have escalated consistently since the 1980s, and both are at record highs.
[Office of Applied Studies, Substance Abuse and Mental Health Services Administration, US Dept. of Health and Human Services, "Year-End 2000 Emergency Room Data from the Drug Abuse Warning Network (Washington, DC: DHHS, July 2001), p. 2.]
Like I said before late 70's America was not a good time.