Am I asking for their number too soon?

The last 2 women I've had conversation with online have ghosted me after about a dozen messages asking if they'd rather text instead of going through the app. I'm not a huge fan of Snapchat but apparently that's what everyone is using these days. What am I doing wrong? The last few that I've met up with in person have offered to give me their Snapchat info after the same amount of messages.
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