We already have hundreds of millions of users. A large proportion of people who use the internet will use Wikipedia in a given month, they use it by reading bits of it. Finding out what the barriers are for the thousands of millions who don't use Wikipdia would be useful. No doubt there are some who are aware of Wikipedia but didn't feel a need to consult an encyclopaedia in the last month, and some who are not currently in the market for an encyclopaedia because they are too young, too senile, or locked up. But research into why people don't use Wikipedia would be useful. Our mission is to make the sum of all knowledge available to all, finding out how we get to the next 400 million people, and indeed what proportion of humanity would use an encyclopaedia if it was available to them would be a great use of research.
Of those hundreds of millions only a tiny proportion, perhaps 0.02% are "active editors", and that on an absurdly generous definition of active (5 edits in one month).
Theory tells us that as quality continues to improve so those readers who fix a typo or some vandalism when they see it have been editing less and less frequently. We know that the edit filters have lost us many of the vandals who used to be such an important part of the raw editing figures of the site (it never ceases to amuse me that the threshold to count as an active editor was exactly the same as the typical vandal needed to get through four levels of warnings and then get blocked). We also know that the rise of the Smartphone and to a lesser extent the tablet has lost us editors, to most tablet users and almost all smartphone users Wikipedia is a read only website not an interactive one. But it would be good to test that as even the most obvious explanation is only a hypothesis until someone has tested it, better still some sort of quantification of those various issues would be very helpful.