if they’re constantly growing their user numbers

All social media needs to constantly grow because attrition. Social media requires basic levels of user ship to be functional, even lemmy. Its a network effect where you need to have certain levels of users for some emergent properties to exist. For example, I speculate that defederation early between .ml and .world was the trigger that will eventually kill lemmy, principally because this results in fragmentation and a reduction in the properties we would get from "more users". Having more users begets more users, more content, more memes, etc. And I don't necesssarily see the defederation as something unneccessary, but what I'm describing is an inherent property of networks. Its not something that can really be argued with because this behavior is consistent across physical, biological, social networks. It just "is" as a property.

So foundationally, you can't sit still on a train moving backwords (which it always is). An organism needs to be constantly recruiting and growing new cells into its network because its also always dying. Growth is "holding still" for any networked system.