Indeed! That's the beautiful thing about this whole decentralised magic. You can use the app (not just mobile app) that best suits you and your interests, and interact with anyone, anywhere.
On kbin.social specifically, federation is currently broken.
The server saw a lot of stress because of the influx of new users and the dev turned on the cloudflare check, so that instance won't kneel down. That broke the federation, other instances cannot go through that check.
Cloudflare can protect against DDOS. In this instance the people are ddosing the server, although not intentionally. There are just a lot of people trying to open the site. So it's not malicious but still an "attack", which cloudflare's service mitigates.
In reddit it was called the "reddit hug of death", when small sites kneeled down because a post got popular with their links in it.
And yes, there are, the second largest is fedia.io
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