I don't specifically go for their Facebook. I'll suggest a number exchange and occasionally they'll be a bit cautious about it. Numbers are better for obvious reasons.

I decided to have a little look at WhatsApp once again tonight and started messaging some girls from my phonebook. This provided a different dynamic when messaging as I guess it's free and more like chatting. The bonus is you can have a cool as profile pic and send pics too, i.e. tell them to send you sexy pics of them