Bluesky, a decentralized Twitter-like social network, is pausing new signups “temporarily” to try and resolve performance issues it’s been experiencing after Twitter introduced limits on the amount of tweets you can see in a day. Even though you still need an invite code to be able to join Bluesky, it seems that the influx of new users has been a problem.
Mastodon is also seeing an influx, but probably smaller and crucially, the signups are spread across many many servers (there are thousands to choose from), so the likelihood of more than a couple getting overloaded now is slim.
The earlier twitter -> mastodon migrations put more stress on the servers because there were far fewer available but things have scaled up a lot since then. And we have a lot more experienced sysadmins and cloud engineers who are capable of scaling up their instances more rapidly at the helm of many of the large instances.