Read research from WMF's Growth team, which existed from 2012-2014 to find ways to attract/retain editors.
Research: Anonymous editor acquisition | Volume and ImpactTakeaways: As of 2014, registered users seem to edit longer and more than anon. users. 6.9% of newly-registered users edited anonymously within an hour before signing up. Users who edited anonymously right before signup are significantly more productive newcomers than the general population of editors.
MassMessage is a tool that allows users to send talk page messages to a list of users quickly and easily. This tool has been activated since October 2013.
This is the summary of findings from the Wikimedia Foundation Summer of Research, a program in the Foundation's Community Department which brought together eight academic researchers from around the globe to study the dynamics of the Wikipedia editing community. Of note: Based on analysis of a small sample of Newbie comments, Newbies aren't good at knowing where to ask for help, and Wikipedia isn't good at spotting requests for help, particularly when newbies talk on their own talkpage.
How the welcome message affects new user behaviorI see a lot of potential in continuing to think about how welcome messages might be developed to appeal to increasingly diverse demographics of editors and to help new users see not only how but also where they can make contributions.