Lemmy is popular nowadays, yet is losing its active users

I think the "infinite growth" here is referring to our expectations of some systems we use growing infinitely, even when the system itself is bounded by finite terms, such as population or hype.

For example, US Social Security works on the assumption that there are more people working and inputting money into it (via payroll taxes) than retired people taking money out. That assumption requires a growing (or at least very stable) working population, as a shrinking working population means that there will be more people taking money out than there are people putting money in. This growth, inevitably, will have to stop at some point. However, many retired people expect, and in some cases financially rely on, Social Security giving them money.

A large portion of the Fediverse have expressed their disdain for such systems, and molave here is finding it ironic that they'd expect this platform to infinitely grow, as well. The initial hype from the whole Reddit shenanigans are dying off, and the platform will soon stabilize, at least until Reddit pisses off its users again.