If you look carefully at Meta's actions in the last few years, you'll notice they're slowly stepping away from the Facebook brand and product. I suspect that they no longer internally consider Facebook to be their main product, giving way for Instagram, which at the moment is a lot more popular and despite the obvious association doesn't have a tainted name the same way Facebook does.
That was my first thought as well. Facebook is simply too different to Twitter in ways that Instagram is actually similar. On Facebook you're blogging your life for others to follow. On Instagram and Twitter people are blogging a little of their own, but most users are there to follow others (usually big names or important people) and to comment on events those larger names are blogging about.