How mopub retrieve reatime eCPM from Facebook and Admob?



Hi everyone.
I’m considering using MoPub Mediation with Facebook Ad and Admob. According to this document, MoPub can optimize revenue by “Real-time bidding”, but it does not specify how MoPub gets the eCPM from different ad networks.
So, does each ad network provides an eCPM API or MoPub generates eCPM from historical report?