You're adding to Facebook's network value by having a Facebook presence, however minimal you deem it to be. You're inducing other people into giving Facebook information on them by maintaining a Facebook account.
Isn't this why they consider Daily/Monthly Active Users and not just accounts? If I have a Facebook account that I never log in to, I'm upping their count of users, sure, but not the DAU/MAU count, which is what marketers care about for reach.
