The issue is not that mobile screens are too small to show more than one step at a time. I've done many landing page experiences on the web that would have only fit on a mobile screen and seen upwards of 75% conversion (for a 16 question onboarding process) and often find that multiple steps convert much better than a single step (asking for email first, then asking for a password, for example). The key is to get buy-in, which is a combination of value proposition and the give-and-take nature of whatever signup flow you are developing. You can't just ask the user for things over and over, give nothing back and expect 50% conversion. You have to make the process feel like a game, get buy in, ask for a bit more, give value early then ask for more. This is hard if you have a social product because you have to have strong social pressure to signup or a good "single player mode" (http://cdixon.org/2010/06/12/designing-products-for-single-a...) or you are dead.