The metric for "it works" is whether people successfully get started and continue to use the platform. For commercial companies this coincides with them making money. Even though Mastodon is a non-profit and does not and has no way of making money from having more users, it is a valuable goal in itself. Each new user decreases the network effects pull of commercial social networks and increases the usefulness of the network for everyone on it. Especially since the userbase is never static as people leave due to natural and unnatural causes, it is important to keep an eye on new user retention.