As Mastodon already passed over 500,000 users and 1,300 separate Instances things are starting to mature and emerge within this new social network. There are Instances already dedicated for animal rights activists, gay lesbian rights, tech, etc where local posts are highly relevant to those groups. Some brands are starting to emerge but with no VC investment and sponsorships it does not take on the appearance of Facebook, Twitter etc networks,
So why the different servers? For one, it makes it easier to run as a non-commercial enterprise if the costs are distributed among a bunch of volunteers. Also, if someone gets bored and stops managing a server, the entire ecosystem doesn't go down with it. Each server can have it own rules and admin, and this also creates safe spaces for some groups.
#animalrights #mastodon #LGBT
Mashabale has doen a nice overview of this maturation at http://mashable.com/2017/04/04/mastodon-twitter-social-network/