Facebook isn’t joining Lemmy. Lemmy is a weird ActivityPub server, it barely works with the rest of the Fediverse because of the design of the underlying protocol. They integrate with Mastodon and other timeline based social media first and foremost.
I personally like the ability to follow people who don’t know how to install Linux on Mastodon. Most servers will silence Threads anyway, so you’ll only see posts from people you follow or posts retooted by people you follow.
Based on what I’ve seen about ActivityPub’s growth, most people came to Mastodon to flee Twitter or to Lemmy/Kbin to flee Reddit. I don’t think any Facebook alternatives have really taken off.