Mastodon

Also the first barrier of picking a server (how it works, the rules of every instance, checking who they federate with) and an app (the will to test multiple apps, learning that to login you have to input the server url manually since most aren't listed in the apps), to the people who read all the things it's tedious but doable, for the rest it's "Which one is the RIGHT choice?" and just stay at the door.

Also servers with poorly written rules don't help (example: mstdn.mx says porn and politics are forbidden, but in reality they allow them as long as you tag then properly).

These kind of posts don't help either, because it makes people feel like they are too stupid to join and rather stick to the known services, but omit all the actual process that someone has to go through.