It's not about "number go up" in the same way Twitter or Instagram is because there's no algorithm to give it a feedback loop.
Both Twitter/Instagram and Tumblr/Mastodon are about getting community/attention from strangers by posting what you want and letting them come to you (as opposed to Reddit or forums where you join existing discussions). But the Twitter/Instagram model relies on an algorithm so it only benefits those who are already famous or who invest in gaming the system, whereas Tumblr/Mastodon make it easy for everyone to find their community of a few dozen mutuals without being buried by the algorithm.