Engaging online students

Having been on both sides of the room so to speak, I am perplexed as to how to engage the "social capital" of online students who don't - just don't. In a classroom I would stand beside them or ask them questions but with a "class" of 100 or so, how can an online instructor keep up with who's sleeping in the back of the room or who's just checked out and is texting their boyfriend? In a physical classroom I have a sense of juggling the plates. Keeping everyone spinning at once, I enjoy the tension. But online? I feel like I'm learning a new language by sitting at the table next to foreign speakers.