There's no Mastodon slump.
Sign-ups continue to increase -- albeit, not at the same rate as in November -- November was a spike.
While Monthly Average Users (MAUs) decreased since November, it's increased again this month (according to @Gargron).
But what's more interesting is that Half Year Average Users (HYAUs) is at a peak.
And we're very close to breaking 1 billion posts per month!
See chart.