How to build a social network with mongoDB?

I'm wanted to start developing a social network like instagram (more or less).
but I have tried to understand how to design my DB (using mongodb) for the main queries.

So I have few questions:

1. how to save the data of followers/following in the db? it should be in the same document of the users or other?
2. how to fetch all the posts for users from his followers?
3. can I use mongodb for building a social network like instagram?