It's both. At its core is a set of protocols that allows different software to interoperate in the way you envisioned. These protocols allows pretty much everyt

It's both. At its core is a set of protocols that allows different software to interoperate in the way you envisioned. These protocols allows pretty much everything you mentioned with the possible exception of video/audio calls. I don't think activitypub has support for realtime protocols. There are many implementations that uses these protocols and that can interoperate today. The most well known of these is Mastodon.