We want out students to like us, of course. In the online environment, though, the social cues are different, and students may confuse friendliness (smiley faces, chatted words of encouragement, etc.) with ardor. It's important that the instructor set the tone and maintain an atmosphere of propriety, whether or not you keep a casual online profile. Consider how your students see you and how you would like to be seen in your online classroom. It's easier to relax after taking a rigid stance than it is to restore an image of toughness once you've caved in.
I generally begin with my online students by discussing the boundaries of our online relationship. This includes how they are to communicate with me outside of class (e-mail and IM), and what rights of privacy I maintain for myself and will respect for them. I'm also careful about how much of my personal information I make available to them and try to establish the guidelines for what's fair play in our online relationships. Since there are institutional policies that apply, I discuss the college or university policies that apply to appropriate behavior online and acceptable uses of technology.
In a class, the instructor is not a peer, in the sense of being an equal of the students. Friendliness is important, but friendship is a stretch: the teacher calls the shots and assesses performance. The line may move in your online sessions, but it has to be drawn.
Discipline is important in the online classroom, just as it is face-to-face. When a student behaves unacceptably, either send a private message to that student immediately or pull them aside in a private chat room to discuss the infraction. Avoid public confrontations, since these smack of humiliation and are an abuse of power that will create more problems than they solve. Similarly, shouting does not solve noise issues online: use the technology to mute students or put yourself in lecture mode.
If you treat your students with respect, give them opportunities to shine in your classroom, provide them with acceptable and fair policies, and avoid negative discussions or complaining, they'll appreciate you as an instructor and reciprocate with positive learning and responses. You'll be their teacher and the learning experience will be a friend they'll look forward to revisiting.