Well, it's true that "what gets measured gets managed." In the Connectors Project, we've seen a slow but steady decrease in the percentage of profiles which aren't connected to the main tree. (Granted, we're not going to see 0% unconnected profiles, because new people are signing up all the time, but it looks to me as if the network effect is already starting to kick in: as the number of profiles connected to the main tree increases, it gets easier for people to make connections, so the main tree gets bigger still, and we end up with a virtuous cycle.)
In thinking about your comments, the thought occurred to me that it might be helpful to have a sort of "Dashboard" screen for WikiTree: one screen with a whole bunch of indicators for how things are going in terms of tree size, active WikiTreers, percentage unconnected, percentage unsourced, percentage uncategorised, etc. That way, people who have been on WikiTree long enough that they know how to handle multiple tasks would be able to log in and ask, "What part of WikiTree needs my help the most today?"