ActivityPub-enabled microblogs are gaining popularity as a replacement for Twitter, but ActivityPub is for more than just microblogging. Many other popular services also have open-source alternatives that speak ActivityPub. Proprietary services operated by commercial interests usually deliberately limit interoperability, but users of any ActivityPub-enabled service should be able to communicate with each other, even if they are using different services. This promise of interoperability is often limited in practice, though; while ActivityPub specifies how multiple types of content can be published, the kinds of content that can be displayed or interacted with vary from project to project.