I'm in my 20s and have worked as an engineer at a top tech company for over a year. I've seen a moderate amount of drug use during that time. I wouldn't characterize it as ubiquitous by any means, but it also isn't that uncommon. I've never heard of any of my coworkers doing drugs at work, but many of them do them recreationally outside of work and occasionally at work functions. This all may sound a little judge-y but this is what I know:
Two of the engineers on my team do drugs with some regularity. One likes to consume MDMA and other club drugs socially when going out with friends (I'd guess he does it on roughly a semi-monthly basis). The other is a regular cocaine user. He's somewhat cagey about this, but I suspect he does cocaine at least weekly. They have told me stories about doing cocaine together with other people in the office including college interns.
At my company there are also multiple managers in their early 40s/late 30s who frequently get drunk to the point of blacking out as part of after-hours work functions. These are guys with wives and kids at home, some of whom are very senior.
Best guess as to why they tell me this stuff is because my coworkers seem to assume I smoke a lot of weed (I don't) and therefore feel comfortable telling me what they get up to outside of work. I also typically drink liberally with them at the aforementioned work events so I'm seen as somewhat trustworthy. It's unusual enough that I'm still surprised when I find out my coworkers do some of these things but try not to judge them for it as I tried a few drugs in college. I'd probably do them too if I enjoyed drugs more.
The attitude of my manager and others towards this kind of behavior is best summarized as "the less I know the better". My boss is also pretty young and definitely has some idea of what young engineers at tech companies get up to. I get the feeling he willfully turns a blind eye so he doesn't have to feel responsible for the indiscretions of my teammates.
My take is that there's a little bit of denial about substance abuse within tech. It becomes very easy to see the drug use described above as normal when you're in an environment where lots of your peers are doing it and everyone is extremely well-paid and well-educated and able to rationalize self-destructive behavior in ways that are hard to argue with. It's a symptom of an industry that puts a premium on people with a lot of raw intelligence and high work ethic but not much self awareness.
If you are interested in hearing more feel free to reach out at the email on my HN profile.