I really loved this simple idea from Coursera.
This feels like an idea we can pinch for communities too (especially superusers).
When members join, invite them to set a goal for themselves. It can be to visit on [x] number of days. Or it can be to achieve a certain rank, learn a specific skill, or read a set number of articles.
Then provide a simple means for members to add those times to their calendars.
I suspect for employee communities and superuser programs, this could be especially effective.