@gruber Some developers have already built mobile apps with Fediverse interoperability in mind.
For example, here's one: https://github.com/caraesten/Fedilab
Fedilab supports Mastodon, Pleroma, Peertube, GNU Social, Friendica and Pixelfed -- and it's open source!
More to the point, ActivityPub was created with interoperability in mind.
I get that these are 1.0 apps -- and I appreciate what they're building.
But the goal of the Fediverse (including Mastodon) is the de-centralization of social media.
GNU social JP is a social network, courtesy of GNU social JP管理人. It runs on GNU social, version 2.0.2-dev, available under the GNU Affero General Public License.
All GNU social JP content and data are available under the Creative Commons Attribution 3.0 license.