They don't need to see exactly what you are doing on Facebook, they just need to know you are going to Facebook so they can make Facebook pay for the traffic (in addition to you paying for the traffic, and Facebook paying their own data provider for the traffic). This is what a VPN solves but regular HTTPS/SSL does not.
How Facebook pays for the traffic (to your ISP, not theirs) is somewhat irrelevant (maybe they are a willing participant and sponsoring it), but ultimately as long as the last-mile ISPs know where the traffic is going to/from they know the two parties they can double dip from.
How Facebook pays for the traffic (to your ISP, not theirs) is somewhat irrelevant (maybe they are a willing participant and sponsoring it), but ultimately as long as the last-mile ISPs know where the traffic is going to/from they know the two parties they can double dip from.