I got asked this by a few different journalists today, so I'm just going to share the numbers. 10,801 people signed up on different Mastodon servers today, and 22,139 since last week.
Update: 49,215 new users across different Mastodon servers since yesterday. Probably a lot of them are actually from yesterday. My crawler only runs once a day so I get a bit of a delayed view of things.
So, that total... By the end of day, it's more like 70,849... That's quite a lot for a single day, isn't it. Federated social media is better than commercial silos, and I'm happy that more people are learning about it.

@Gargron Yeah, it scales pretty well, at least on my side. Some other instances might have performance issues.

Over here it's like +1000 users or >50% more within the last 4 days, according to https://the-federation.info/node/nerdculture.de

Still room for more. CPUs are doing well, storage is ok, etc...
So, 👍 for the performance of Mastodon as a software! 🙂

@ij Thank you for providing the service! 😀