Relations.js

/*
userData = {
	'person1' : [1,2,3,4],
	'person2' : [3,4,5,6]
}
*/

First loop over all the userData once and construct an index.

index=[]

for user in userData
  for n in followers
    if(!index[n]) index[n]=[]
    index[n].push(user)

This is O(n*m), where n is users and m is avg followers.

Next, loop over userData again, and construct your results

results={}

for user in userData
  result=[]
  for n in user.followers
    result[n]=index[n]
  results[user]=result

This is O(n*m) again.

Now your results looks like

{
  'person1': [,,,['person2'],['person2']]
  'person2': [,,,['person1'],['person1']]
}

...but if you want you can invert them to be like you want.

function returns

{
	'person1': {'person2': [3, 4]},
	'person2': {'person1': [3, 4]}
}
*/