Oiknine says Apsalar's own numbers suggest iAd's share of spend is even lower than eMarketer estimates-- down below 5%.
"Apple's attribution approach… will not help them narrow the gap," he said.
But iAd has the upper hand over Facebook in at least one area: data sharing. Advertisers can combine iAd attribution data with in-app performance data, notes Slaven Radic, CEO of TapStream. He says this gives "more color" around the total impact of mobile advertising, but it's a no-no for Facebook mobile advertisers.
Facebook has long frowned on the mingling of its data with outside marketing systems, among other practices, and is serious about enforcing restrictions. In February it expelled from its measurement program two companies who were found to have violated data collection policies.
"With iAd, at least we can tell advertisers what kind of engagement they're getting," Radic said. "Facebook doesn't like to share this data far downstream. You're very limited in what you can do with it."