Twitter is another great tool for subliminal teaching. In my past experience, I have used twitter for both, reminders about exams, homework assignments, and other class related deadlines, and also for exam reviews through questions and answers sent via twitter.
Since twitter is limited to short (140 character) messages, it requires careful wording. Normally, when I read/review the chapters, for my preparation, I make brief notes (electronically) about messages I want to tweet, so later on, I can tweet these by using copy-and-paste.
Take a look at the "Higher Education Trends and Statistics" page at Cisco. A couple of the applicable facts listed in the Mobile section are:
97 percent of U.S. college students own a cell phone, and 79 percent own a mobile computer
73 percent of Americans ages 18 to 29 who own cell phones use them for data and communication activities other than phone calls
What this means is that a large percentage of students are capable of receiving tweets on their cell phones. If the content is kept useful and interesting, the tweets will be read. The time spent reading/digesting this content is neither class-time, nor homework. It is an opportunity to take a chunk of students' free-time or socialization-time and convey an important concept or a question to redirect students' thoughts to course material.
I look forward to additional thoughts from any of the readers of this blog.