Hmmm.. it totally depends on the circumstances. How close are you? Do you ever see her off the net (at school or something?)... If you see her at school and talk outside the internet, it wouldnt be so weird. But I wouldnt go jumping into any heart-felt confessions if your relationship isnt v. personal. She'd probably reply by saying "But you dont know me, so how can you like me?". My advise would be just keep talking to her, and make an effort to make the conversations personal, or at least extend their duration. And if you can, maybe meet up on the weekend to see just how well you get along without the net, because its extremely easy to become close to someone over the net, but be lost when it comes to face-to-face conversations.
I'd say just establish more of a relationship with her before you tell her you like her... you never know, after getting to know her better, you might not like her!
... seriously! Talk about yourself naked, it forces her to think about it! 
