Research:Mobile editor engagement/Newly registered users
The following plots show the increase in daily account registrations driven by mobile (mobile signup calls-to-action were released at the end of July 2013). As of April 2014, the mobile site is responsible for about 20% of total account registrations. Mobile registrations also don't appear to be subject to seasonal fluctuations of the same order as desktop registrations.
Daily new accounts, before/after launch of mobile signup
Daily new mobile accounts