[hnaccount_rng]

You can't though. The problem is, that humanity is a web. Not a set of communities (at least on the scale of 1000s of people). And since those webs overlap you will either need to solve the overlap problem at the boundaries (taking engineering effort) or you will end up with essentially one big shard again. On the other hand, you really don't need to change anything on the backend. Simply limit the number of "tier 1" friends to 50, have a "tier 2" category for your 1000 and put everything else in "acquaintances" and split engagement between those.

The problem with that though is: You will generate an enormous amount of social friction "why am I tier 2, but (without loss of generality) Karen is Tier 1?" and reduce monetizability. So truly nobody will feel happy about those restrictions. And since it doesn't solve any engineering problem you run into (see above) there is no one incentivised to build such a thing. (Ironically this may not be completely true, given that this is pretty much how Chinese social media apps work. So maybe states [or at least power structures] are incentivised to build such a system)