Is there a reason to ask for a number at all? If you're unsure about whether someone is interested asking if you can add them on eg facebook seems much less pushy! Then you can message them the next day saying "lovely to meet you." If they're interested they'll reply.

The straightforward answer is that those PUA folks who do lot's of approaches find that the chances of getting a date are higher when they ask for a number than when they try to connect over facebook.