The overall lull has ended for me! I just got my first invite today (and am SO thrilled about it!) . . . and I certainly hope this is just the start of good things to come!
However, I was complete at most places later in the app season, so perhaps there is a natural lull for the earlybirds at this point. You're right in that it has seemed quite quiet recently . . . and I think I recall the same thing happening last year. And I would bet that stat-wise, less interviews go out from Dec-March than from August-Nov. What I also recall is a smaller wave of interview invites going out in January/February . . . and people being quite surprised/pleased to receive those. I have a feeling that schools perhaps table some of their already reviewed apps right now to wait for any amazing stragglers who may still be submitting. When the schools' deadlines for secondary submission pass, that's when I think they go back, re-review, and send out whatever remaining invites they plan to send.
This is all just theory, though!! I would love to be proven wrong and get a few more invites before the month is out.