On POF, I've noticed asking for the number on their is futile. Only if they're extremely attracted to you or my profile, have I gotten numbers off the bat. The majority of the time, I message them about 4-5 times and on the fifth or fourth, I'll ask for their Messenger, E-mail, MSN or Skype. Now, yeah it sucks to use these applications. But it is a handy-perk if you're going to build some forms of rapport and comfort with a chick off of there. After a bit of chitchat, I'll exchange numbers on a high-note. Ideally, I'd want to get her on a high note when she gave me her e-mail. Stop asking for numbers, start asking for messengers and emails.
You see, girls like to prolong the inevitable. Especially on MSN, try not to catch yourself building too much of a "connection". Lots of teasing, knicknames, funny scenarios of you and her, taking her for granted, Big brother & little sister flufftalk. No interview questions. Let her ask YOU the questions and be hot and cold with them, less is more. You don't have to worry about building huge amounts of rapport, leave that for the phone and even then, you want to do too much of rapport-building, preferably you want to get the date pretty much the same week. Be dominant and leading. When she is on a high-note, ask for the number and that you'll text her in 5 minutes because you're logging off and going to get something to eat.
She's still hesitating on Instant Messengers and E-mails?
Don't sweat it. Tell her "Haha. You should totally play hard to get, well I'm going to get something to eat now. Later". Be hot and cold, you see her online. Speak to her, by this time, she should figure you're a cool and calm collected dude and feel comfortable soon enough to give you her number. Be hot and cold with her. Why? shows you don't care and not another desperate chump online. Hot and Cold is basically push and pull. If at this point, you haven't got her number, she doesn't say whats up or she is a stale contact online, NEXT. This has worked for me, others may disagree and tell you to ask for the numbers on POF itself.