Well…
If you follow the link to fedidb, refer to the “mau” monthly active users. Do some brief math and realize that lemmy.world accounts for about 50% of all active users
Email market share is harder, but many estimate that Gmail accounts for over 40%. Many many orgs use Google apps to make custom branded gmails with their own domains too
This is the typical “business power law” that states that the top player should control about 50%, the second player about 25%, etc. This is just kinda how the world works
You may find it funny to learn that Signal’s early development was funded by an arm of the govt that focuses on distributing ideology worldwide – US Agency For Global Media (Radio Free Asia and Open Technology Fund)
You may also find it interesting that Trump eliminated this entire agency about two months ago
Why did the USA run all these programs and fund privacy software? Ostensibly to help people in oppressive regimes communicate. To broadcast alternative media