After searching the forums with google, I see there are (at least) 5 outcomes to thank you notes written to Columbia's admissions:
Of the 3 "personal" responses, they seem to break down like this:
"very much in mind." Seems to be reserved for those that interviewed with Frantz and also appears to be the "worst" response based on past years.
"very strongly in mind." Both accepted and waitlisted students got these. Nothing to infer either way?
"very strongly in mind.... great personal pleasure... you would be an asset to P&S." This message (again based on previous years' posts) seems to be a positive sign, but some that received it were still waitlisted.
I sent a note and got "very much in mind" FWIW. Just in case you think I'm thinking about this too much, do a search. There is literally a decade of speculation about this, and you can find theories about the particular wording of each letter from as far back as 2001.