Originally Posted by Konda

A huge portion of the ecpm in adnetworks comes from shady mobile carrier offers that use 1click direct carrier billing practices to scam the users out of money. More and more mobile operators are now making their rules a lot stricter in regards of content and the billing process itself, so that's probably why you see it dropping and will probably continue to drop as the telcos make stricter and stricter rules. Try to break down by country and device and by carrier if you can and you will see where the big drops are.
Like I heard recently Vodafone in a big European country changed the direct carrier billing process (from 1 click to like 3 clicks), those things have a huge impact on your ecpm because that's where a large portion of the income comes from.