How do you get the attention of your audience? Using an attention getting activity or puzzle to engage students in active participation. I must admit, I have not thought about purposely designing a virtual 2x4 within an online course. I have made the assumption that they are geared towards completion of course and working through the lessons. I do use personal reflections of particular readings to facilitate ideas and thoughts within themselves and others.
With my face to face classes, having a gathering activity that gets the students engaged in dialogue from a "puzzle" or problem solving situation. Within the virtual world, this could be simply a piece of media that has the elements of a puzzle or something to capture the attention and provide a "thinking" time that requires a response.