In one sense there is no universal feed in #nostr as far as I can see. You see what your relays show you. That's the first filter, applied before any other.
That's probably true on Facebook and twitter too. It's just more transparent here.
Discussion
is there a hashtag so lame?
There are some very popular #hashtags such as #asknostr but none that deliver a firehouse, not that I know of.
Unless I'm misunderstanding you?
Primal doesn't display to you notes that live on your selected relays. On Primal, you ONLY read from their caching relay.