They're not fragmented or segmented at all. It's like email—just because you have an account on gmail.com doesn't mean that you can only email people who use GMail, you can email anyone who supports the same protocol. In the same way, users with an account on any Mastodon server can follow any other users who support the same protocol, whatever server they happen to be on. (Modulo moderation—many servers with lax moderation policies may find themselves blocked by other server administrators. Again, just like email)