Hey there, I'm a friend of Cara Delevingne, and I can tell you that she downloaded Snapchat but she didn't really see much use for it, so she kinda doesn't bother adding people on there.
Like I'm one of her best friends and she probably sends me like 3 messages a year on Snapchat at most.
If you want to connect with her on social media, then she does use Ask.fm though.
You will find her here:
If you ask a question or just leave a cute message then she will answer you!