I'm not talking about bolted-on "social" features. There are entire multi-billion dollar industries built on categories of software that did not exist in 1990.

Google Docs isn't merely a word processor (spreadsheet, etc.) with social features...the "chat" is ancillary to the real benefits; it's an entirely different way to work with documents, and that's one of the examples of things that's extremely close to 80s/90s tech; it looks just like a word processor, and people from 1990 would know how to use it. But, it's not the same thing, and before it (and some other online document tools) came along, Word had extremely limited sharing capabilities (requiring ridiculous intranet servers to host the shared docs, and it was basically the same as passing it around via email only with slightly better revision control). That's as close to a traditional app as you can find, and it is still 100% more valuable for being on the web.

What would [ Youtube, facebook, Google Maps, Amazon, craigslist, Netflix, etc. ] look like in a desktop app, and why didn't they exist before they came to the web? The web is a unique (so far) platform with distinct benefits that aren't available to apps in that past. The reverse (desktop/native apps could do things web apps couldn't) was also true until relatively recently, but that's changing, though I don't think the interesting work is in porting native apps to web apps...people will do it, because they can, but the interesting work is in the new things made possible by the web itself.