When talking to a girl, asking for her phone number is a way of expressing interest in her.
You don't want to act too quickly or you come across as desperate. For example, asking her what she does for fun on her weekends is a great question, because it gives you yet another opportunity to relate to her. Nowadays, "We met online" is a common response from couples who are asked how they got together. However, it's important for me to meet someone in person to really get to know them. On the other hand, she might have hot friends, so consider inviting her to your next party.