There is nothing harder than maintaining focus (both the student and your own) when there is no eye contact, low attendence at Live Chats, and little communication with students. One way is to continutally change your presentations to keep them fresh and to keep you on your toes mentally. Second, continue to bombard your absent students with information - be the 'drippy facuet' in their lives.
For some the ability to ignore all those messages gets weaker and they actually start to participate. Finally, there is always the 'shock' treatment method. Some students will only become engaged in the learning process when they realize that your class is not the 'pass through' they anticipated at the start of the session.
One of the most difficult things to instill in online students is the sense of urgency and timeliness regarding their assignments. For some the light goes off early - others become discouraged and start to fall behind - those are the ones that you have to work hard to retain and rekindle that spark for learning.
In either case, it is up to you - the instructor - to light the spark and keep fanning the flames.