Twitter::Cursor is Enumerable; just call to_a

There's certainly no need for you to write your own recursive function.
The whole point of the Twitter::Cursor class is to encapsulate the logic
of how to iterate through results.

That's why Twitter::Cursor#next_cursor is a private method. As a rule,
you should never need to use the #attrs method on any Twitter object.
I've made the raw data available for contingency purposes but it's
almost always an indication that you're doing something wrong.
Fetch friend/follower IDs in separate threads

If the first user has a large number of followers (over 5,000) and the
second user has relatively few, there's no reason for the second user's
HTTP request to wait for the first user's HTTP requests to complete.