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I can't really think of any outside of youtube. There's local search engine (Yandex and some lesser ones), there's local social website (vkontakte). Russians don't talk English well and mostly use Russian websites which are, obviously, mostly hosted inside Russia. Wikipedia is another example, but, funny enough, Russian government wants to develop a curated alternative and already have money to fund it (basically web hosted encyclopedia).

YouTube would be one prominent example; Wikipedia, Google search & hosting, Twitter are all popular.

There are also media of a more liberal nature who can’t comfortably host in RuNet / being registered as Russian companies, so they exist outside. meduza.io being a prominent example

I would assume any foreign website (Twitter, PayPal, Facebook, etc.) would need to be co-located within Russia to continue to operate. Even then, it would be cutoff from updating or querying any foreign databases.

I can imagine online banking with foreign entities would become a problem as well.