I've confessed to women in online relationships in which we were very far away from each other. It turned out pretty well, actually. There is some level of emotion depending on how close (personally) you already are when you're talking to them online.

Still, It'd still be better to confess in person if possible. There's a lot more emotion behind it then in just text being sent.

Besides, confessions aren't really that hard to do itself, or at least they shouldn't be. If you feel you need to confess, it should be at the right time, and right when you really mean it and you have to get it out. Timing is the most important factor, really. And since it's a confession, you don't want to fuck up mid-way. Look at that person straight in the eye and speak from your heart (LAME but true).