Well look, I don't have enough insight into the design or backend for Lemmy or mastodon, but Facebook has heavily invested into their network, and likely aims to grow.
How could they do that? All of this seems blockable on the client end (meaning I'm not good/shitty enough to work at Facebook) but imagine:
an algorithm takes a selection of high ranking fb posts and cross-posts to Lemmy, far faster and more frequently than regular users. Oh, you'll need to login to read.
threads could wholesale repost other users and their comments, but behind a threads login wall
Basically do some scummy behavior using our public statements, questions and comments, all to get more attention devoted to what's happening on their site (and its associated ads).