If you deny them Telegram, Facebook, Parler or any other online platform, people would gather offline and have their echo chambers there. Just as they did before Internet. So you're basically saying 1st amendment harms society, not online platforms.
Edit; As I said I’m a libertarian and a group of whatever people talking what ever where ever is totally fine with me but manipulating people with intent is not protected by the First Amendment And shouldn’t be confused with free speech
Was it a problem a couple centuries ago, when offline media emerged and blossomed? It was the same effect, just smaller scale. Yet, free press is valued as essential for democracy.