WHO.
Errors connected with the use of this word are very common, even amongst good speakers.
"Who did you see?" "Who do you know?" and "Who did you hear?" are wrong: whom should be used, for it is the object of the transitive verbs, see, know and hear. Who in such sentences as: "Who are you looking at?" and "Who are you writing to?" should likewise be changed into whom, for it is the object of the prepositions at and to.