Deplatforming works. A few people follow to other sites, but most don't. It also becomes harder to attract new cultists if one is shut out of major services.
Yes, it is one of the methods used by Russian government to silence the opposition. Though the platforms are usually privately owned, they all 'independently' decide that some content can't be allowed on them.
This is not true. Russian government does not dictate FB and YouTube (which it does not own), and they would have to force ISPs to block these sites if they wanted to ban the content. I doubt ISPs would ban them themselves.
Do not be so fast with 'not true' claims. Russia has at least three social networks, each with audience bigger than russian audience of Facebook and Twitter combined. All three remove anti-putin groups, etc.
the third one is Moi Mir (https://my.mail.ru), an extension of by far the most popular email service in Russia. Mail.ru corporation actually owns all three of them.
VK absolutely does remove anti-putin groups and events. [1]
As has been so often quoted on HN, "The 'Net interprets censorship as damage and routes around it." I see no reason for things to be different this time.
Same will happen with the US. Sad.