I'd be surprised if they're not logging the amount of data used (size of message), probably also it's type (voice or video call, or just text - possibly with some sort of attachment), not to mention location data for each participant, and whatever else they can glean - frequency of calls/messages between users for instance, and how that relates to other data FB know about - all sites visited that have social media buttons, as well as what anyone posts directly to their feed.