How does inviting friends from other social networks work?

Question

Google

Hello!
Need help following. question.
In VK, on ​​the friends search page, there is a functionality to search for friends from other social networks.

What data does it request through the API?
Everything is clear from Google. There it is possible to request the user's contacts and then do whatever you want with them.
I didn't find anything on Facebook. It is possible to get only friends from the application. In my case, it's useless.
Twitter and classmates are not particularly interested, but if anyone is familiar with their system, I will be happy to help. Might have to use them in the future.
PS/ The task is to invite the user's friends to the site (recommendation social network) at his request.