The approach so far seems to have been to spread the load over as many different servers as possible and disable sign ups if any individual server gets overload

The approach so far seems to have been to spread the load over as many different servers as possible and disable sign ups if any individual server gets overloaded. Also from what I've read, explosive growth has been an explicit non-goal of the Fediverse. Of course, it remains to be seen how this all will cope with the current situation when Mastodon is pulled in the spotlight. After all, Bitcoin was also a small community of enthusiasts for almost a decade. The real challenges only started when they went big.