Does this take into account people who are upgrading to a new Android phone from an old one? If not it could really impact the metrics if one is looking at this as a measurement of new entrants into the Android ecosystem.
This metric is used is because Apple uses it for their iOS devices in keynotes. Surely both platforms could use "app store accounts with their first device added" per day as a metric if they wanted to. And for Android, you'd want to know what version these activated devices are - no reason a 1.5 device couldn't still be activated today even if the app ecosystem has seemingly moved on.