If all else fails, there's absolutely nothing wrong with coming right out and asking a girl for her number.
One of the perks of online dating is that you're not communicating in person or conversing in the traditional way.
Equally, waiting too long to ask for her number could come across as disinterest.
She may conclude that you just want to be online friends and turn her attention to another online dater.
When she asks you the same question, say something like "I enjoy it, and I'm really pleased to have met you.