Post is already implying that their monthly active users (MAUs) are at near parity with Mastodon’s.
However:
1. Due to decentralization, any report of Mastodon’s MAUs is speculative whereas due to centralization, Post’s are not
2. Whatever the stats on MAUs, there’s more conversation happening on the Fediverse than on Post
Post’s MAUs are not comparable to Mastodon’s MAUs.
So from a functional perspective, what does it matter how many people have a Post account?
Most people on Post don’t say anything. Mass media conglomerates might like this about Post—since they generally don’t like it when the “riff raff” have opinions—but I don’t like so-called social media focused on consumption.
Personally, I write to converse—not to feed “content” to a hypothetical lurker.