Until the new Student Center is finished and all students are forced to pass through at least the mail room at least a couple times a week, traffic is not conentrated on any one point on campus. Online traffic is also not as concentrated on the WSO Blogs as some might think. These are all they various ways to let people know you have an event.
Best Places for Posters
Places where people eat are necessary:
If you are advertising for classes its best to focus on dorms. First years = first year dorms, sophomores = Mission, juniors = Greylock Quad, seniors - good luck.