How would I extract data from social media apps?

I just started dipping my feet into iOS development, and I was wondering how apps like InstaFollow and SnapHack get their data from Instagram and Snapchat.

For example, InstaFollow knows who unfollowed a user. How would I also get a user's information (followers, following, pictures). I did my best to search the internet for an answer but didn't know where to begin. I stumbled across something called Open Authorization. Should I look more into this?

When searching the internet for an answer I can across a Snapchat blog saying how snapchat does not have an open API (or something along those lines).

Basically I guess you could say my question is how can one incorporate data from other apps into an app you create?

I know this question is quite vague, but if someone could direct me to where I could further research this, it would be greatly appreciated.

via Chebli Mohamed