I agree that getting to know students is very important. It's a bit different in the online setting since their names are attached to every post, assignment, etc. However, I still think it's important to call them by their preferred name (Ex. refer to Christopher by Chris if that's what he likes to be called by) and also to use their name when responding to them (For example, instead of great point, say great point Chris, ...). Finally, I'm really going to work on getting to know more about my students individually since this is tougher online (in my opinion). When they introduce themselves in the beginning of class, I plan on jotting down a couple things about them that I can refer to throughout the term (such as "pregnant").