Historians communicate with one another in many different ways. Obviously there are conversions in classrooms and at universities, and I imagine some subfields have specific sites or ways of communicating online. I subscribe to a listserv of scholars who discuss Pacific issues, which is very active, and a number of H-Net listservs as well -- some of which are more active than others. Communicating through publication is the most formal way historians exchange ideas, but all of these formats are complementary -- no online forum could possibly replicate the rigor of a more deeply researched piece of writing, nor are publications alone adequate for accommodating all the conversations historians need to have with one another.

And then there's Reddit, of course.

To second everything that /u/VemeersHat has said and to add that from the perspective of one who is undertaking her degree in the UK (so the system may be a bit different from elsewhere, particularly in the US) not only is my department active with regards to collegial discussion and collaboration (formal and informal) but we host research seminars, symposia and conferences, and send scholars to other institutions for these things, all year round. My institution has also recently started encouraging staff and postgrads to disseminate our research in blog format and I too subscribe to a number of lists both on H-net and through listservs maintained by various bodies directly concerned with renaissance, medieval and early modern Scottish and British history. Book reviews and journal or book publications are often the result of previous debate and discussion with one's colleagues and supervisor(s) after several drafts have aready been completed and reviewed.

I'd like to add that professors also participate in reviewing the history programs of different colleges (not sure about grad schools). I go to a small liberal arts college in Florida and a professor from Connecticut (can't remember which school right now) came and I showed him around and discussed what we'd like to see added or changed within our department. It was interesting to be a part of, and I always wondered how/if departments ever got external reviewing or communicated with others across the country. This, of course, in addition to hosting conferences, paying for talks and putting on events for the public!