[moxie]

"Various activist groups" setup their own SMS service?

If all you want is for a few hundred people to be able to communicate, you don't need a federated protocol, you just need a VPN. It is much easier to use a centralized service and access it with a VPN than to use a rotating set of hosts across successively blocked federated services. At least when the VPN gets blocked you don't have to rediscover your entire social network when you start using a new one.