Was hesitant to post this because it might simultaneously freak people out unnecessarily or potentially give people false hope, but since I see a lot of the same questions on this:
The way Columbia admissions were explained to me, is that all applications are reviewed initially. If reviewer one wants an interview - boom you get it. If interviewer one rejects you, you are reviewed a 2nd time by someone else - if he wants an interview, boom you get one.
So people who haven't gotten an interview yet, it doesn't mean you were dinged, it just means:
1) they've done the first reviews but not the second reviews...if you were rejected a 2nd time, you would've gotten an email
2) It's also highly possible they segregate their queue by expertise and by stats. If you have a certain profile, it could just be that pipeline is more "clogged up". A lot of schools review high GMAT's first.
With that said, I think a good assumption to make, is that if you get a interview request after a long wait, it probably does mean you are on the border, and you really need to do well on the interview.
Please note I could very well be wrong on all of this, but this has been my impression from talking to people and going to the speaker events.
Good luck!