In Moodle2.1, If you check the activity report in the course, you will find that all wiki's log is disappeared. I tracked this problem, and found out that when add_to_log function is called in wiki module, it don't add course module id to the log, If your course has more than one wiki, you will not be able to distinguish these logs.
I submitted MDL-29425 to solve this problem. If you has the same problem, please vote.