@pluralistic that is still the situation: there is not a serious peer service provider operating those services in the network for the microblogging modality.
there is nothing preventing it: we support adversarial interop in the live network today, but no "adversary" has emerged.
hobbyists have run full-scale components as proofs of concept. there are fully independent apps (like smokesignals and whtwnd), and independent projects which index/store the full network (like clearsky)