Well, the fact is that just about any drug one desires is available on campus, but drugs are not a highly visible part of campus life. Tons of people drink, and many smoke pot, but there are also select pockets of the student body that use other drugs, as well. Ecstasy is big sometimes, as are shrooms and acid. Cocaine is available, but only the richest use it with anything approaching regularity. Lots of students use pills as study aids, at least that's what they claim the pills are for, but harder stuff like meth or heroin is almost nonexistent on campus. The conventional wisdom is that if you want to find a drug or a group of people who use a certain drug, you'll probably be successful, but if you don't, except for beer (and sometimes pot), you really won't have much experience with other drugs. Users tend to be somewhat responsible, and very circumspect, which is largely because the College is incredibly tolerant about pot and alcohol and hard on anything else.
Drugs aren't a necessity for a social life at Pomona; more often than not, they're an escape from a sometimes overwhelmingly-present social scene. Alcohol and pot are definitely mainstays of the social scene, but not the entire social scene. They can certainly be avoided if you want (especially if you live in a substance-free hall), but other drug use is generally geared toward escapism. The very fact that some students can attend Pomona for four years and be active and social and still have no idea of the extent to which drug use pervades the campus is proof positive that it's not generally that big of a deal. However, the grade would be higher save the prevalence of alcohol on campus.