There are no users “exploring open source alternatives.” Have you seen the Lemmy signup flow? It’s a complete shitshow that probably turns away 95% of people to begin with.

Facebook almost certainly doesn’t see Lemmy and Mastodon as a threat or competitor. They adopted ActivityPub because it’s nice, and they’ll move on as they need to scale, and Lemmy and Mastodon will continue to survive as they always have.