I can't see any justification for collecting the actual text of my messages.
But if I'm digging for justifications, I can see some benefit to me, the user, of collecting aggregate stats of who I contact regularly. It could be used to decide what content to surface in my feed. Or whose birthday to remind me of. Or which names to suggest when I create an event invite. They are a social platform, after all, so knowing who I socialize with seems relevant to making that work better.
Not that it isn't creepy, but if the question is whether there's an actual benefit to their having the data beyond just targeting ads, it's somewhat plausible.
Other services ask you to give them your email username and password, so that they can scrape your inbox to discover your contacts. I think LinkedIn used to do this, but they appear to just use SSO with the largest providers now.