Hey Mogga,
This depends on a few factors and how you primarily use social media is the main factor. I very rarely use social media and as Fuck This has said you primarily want a very intimate way for girls to contact you through your phone (number). When I meet girls in person the only option I give them is a phone number and basically won't accept anything else unless it's through social circle and you somewhat have to oblige, however if I'm interested in them I quickly let them know that I don't use (Name SM App Here) and get their number instead.
However you can convert girls through social media, and a lot of people use it as their default to communicate because that's their preference. Again it's not how you will invest in them but how they will invest in you so you have to get them to switch to what you prefer. For me that's by my phone (number) with text and calls. (We're all aware that FB messenger works like text and even lets you call and face time but that's beside the point).
I'd recommend during meeting them you push for the number and let them know that this is how you want to communicate with them. If however you get home one night and girls have added you through social media there's no harm in building rapport if needed and then 'converting' them to your phone.
There are certain setbacks to this as they could be using social media to run a background check/screen you or they're just after collecting 'fans' to which the only advice required is don't be a fan. You'll be friend zoned faster than anything. Otherwise just run it how you would through text.
SilenceintheSnow