I think something like this:
I think this makes the most sense to me because if I go looking through log files, I know what channel I want and maybe the approximate date. If I'm in the same channel on multiple networks I personally cannot usually remember which network the conversation was on, so it's less important as a sort criteria to me.
Maybe an added option would be to have a default of some sort and let the users be able to change it later?
Edit: Minor typo fixing