XMPP is truly decentralized with no single point that collects this kind of data. There have been some attempts over the years, but they always failed to capture more than a fraction of the network. A recent one is more artistic in nature, probably realizing the futility of it.

The popular Conversations app has sold 100k+ units on the PlayStore, but since you can also legally get it for free, that is probably only part of the actual users.