Once you’ve established a good rapport with your match, ask to add them on Snapchat. Snapchat is the most casual app to add someone on for a first step.
Facebook may be too personal at this stage due to all their friends seeing the new connection and Instagram is basically saying “I want to stalk all your photos”.
Snapchat is an easy way to send pictures of what you’re getting up to in the day with a cute personal touch, and it is there to let the other person know you’re thinking about them without needing a reply.
If they don’t have Snapchat, encourage them to download it, as it’s free.