You are right that it isn't "just web dev", but at the same time the rest of the statement still holds. There's lots and lots of large-scale (mostly-)web services, and running one certainly isn't that groundbreaking anymore. If you really want to work on one, it doesn't have to be Twitter.
I imagine theres lots of groundbreaking stuff to work on at Twitter. Lots of opportunities to apply AI, do cool distributed systems research, build specialized hardware to optimize video, build new types of databases, apply computational social science (all things facebook does fyi). Its far far more than "just web dev". Now you can just stop working on your products and just hope the website stays alive but not sure its a good plan in the long run
