Since my girlfriend and I get close online (even though we had known each other before), I can tell you what she used one me:
1. She would often start conversations online, and use a lot of exclamation points and capitalized words, as if she was excited to talk to me. Example: "HEY!"
2. She also started calling me terms of endearment. She started calling me "honey" out of the blue.
3. Used a lot of emoticons, such as <3 , :) , etc.
4. Long responses. If I'd ask how she was, it wasn't a "good" or "nm" , it was detailed, like; "I went shopping with my sister this morning, then studied for my medical ethics class, and now I am watching tv!"
5. Then she started to get more serious. Our conversations became deeper, and she started to show sympathy for me.
6. On that note, she started to ask about previous relationships, if I was in one, etc, as well as talking about her past relationships.
7. She asked a lot of questions about me, my life, my future goals, etc.
8. She kicked up a notch even further, she started to hint of a future of us together. I thought she was kidding, but later I learned, she was serious.
9. Then finally, one day, she dropped the bomb; "I feel close to you" and "I want to be in your life"
Also, our conversations would last for hours. It wasn't like a 15 minute thing, where she would then stop talking and sign off, it would last for 5 hours or so.
That's when it all took off, and now I have a wonderful relationship with her. In fact, we don't talk as much online anymore, as we now see each other often and call.