the-federation.info chart showing active users across the Mastodon network went from about 500k in October to a peak of about 3800k in December, then about 2600k at the beginning of February. This figure is aggregated so it doesn't tell us which users are active. They may be earlier returning users or whatever.
https://mamot.fr/system/media_attachments/files/110/691/618/053/937/919/original/b5801971573511d9.png