Pixelfed is growing at a rate of 20,000 active users per day. (source).
Mastodon has recently been growing at 5,000 to 10,000 active users per day. (source)
Those are active users, i.e. actually posting or liking/favoriting. Since yesterday pixelfed.social had to turn off its global feed. Mastodon.social still has its global feed but it doesn't seem to be getting pixelfed posts and isn't updating automatically. Maybe it's good that the press is ignoring them. If they grew any faster they'd become unusable.