I've found the opposite case when advertising my app on both networks. Conversions were about the same, but cost per click was about 4x on google. Its a $4.99 one off payment app, so its possible different revenue models find one or the other better for their use.
To be fair we are not an app and are targeting a demo 35+, so that could be an issue... We are also doing some decent volume (few K+/day on both), if we were doing a coule hundred $/day I'm sure we could get FB working as well, but finding it VERY hard on large volume. Not to mention banner blindness sets in after a few days on FB so we are constantly changing ads.
