I think I covered this topic on the online sarging area but I'll post it here again since you asked specificaly, get a good convo going (like you did) but absolutley add a time constraint, 30 minutes is WAY more than enough time to get her Net-Hooked. The only problem with a net-hook is at this point she isn't thinking of you as a real person, just some dude from facebook.
after the time constraint ignore her for a few days, after the ignore period (2-3 days) hit her with an opinion opener, make it an opener that you can neg no mater what she chooses (also explain that you're in a hurry again and set another time constraint) after she responds to what you ask say something like:
"Wow, that's messed up, I cant believe you would think that,
you're terrible! lol

"
This way it is a light hearted neg (negs are hard to pull off online... no inflection/body language + she has time to reply negativley to it, so keep it light)
use the rest of the time to have a random convo then when the time runs out tell her you are really in a hurry and you need to leave. (seriously my rule for online sarging is STRICTLEY 30 minutes max try not to break it if possible... cut her off if you need to)
wait a few more days and tell her that you enjoy talking to her and would like to continue to do so, but, it would help if there were an easier way to contact her because you get busy and don't always have time to use facebook, follow that up with:
"What's a good way for you?"
This way you are apearing to give her a choice but she really only has one option,
you will more than likely Number-close from here, and giving you her number was her idea! (better yet)
I hate online sarging with an undying passion and have used this technique dozens of times and have had a VERY high success rate
Hope that helped

,
-Lush