Tumblr will add support for ActivityPub, the open, decentralized social networking protocol that today is powering social networking software like Twitter alternative Mastodon, the Instagram-like Pixelfed, video streaming service PeerTube, and others.
once an open-source program gets enough attention it achieves “critical mass”, and proprietary software can never catch back up.
They probably see the inevitable and are jumping on the train before it leaves the station