[da_chicken]

We do, but "who" is acceptable in both tenses. There's no reason to ever use "whom" because it's the only one you can use wrong.

"To whom did you give the book?" is more often "Who'd you give the book to?" complete with the similarly forbidden preposition.

[seabass-labrax]

Yet, among those with whom I associate, "Who didja..." is more common than the semantically equivalent elision "Who'd you", probably because it is ambiguous whether the latter is in the past or present tense.

(For context, I live in south-west England, have an RP accent, and 'whom' was genuinely the word that felt most natural to me when writing this post.)

[lupire]

People speak of the future conditional so rarely, it doesn't affect linguistic evolution. "Whodja" being shorter and less stuttery than "whodidja" is a much more powerful driver. Like "Wensday", unless you actually need to disambiguate past from future.